U.S. patent number 11,150,464 [Application Number 16/608,035] was granted by the patent office on 2021-10-19 for optical scanning device and method of adjusting optical scanning device.
This patent grant is currently assigned to MITSUBISHI ELECTRIC CORPORATION. The grantee listed for this patent is Mitsubishi Electric Corporation. Invention is credited to Yoshiaki Hirata, Kozo Ishida, Takahiko Ito, Yoshitaka Kajiyama, Nobuaki Konno.
United States Patent |
11,150,464 |
Hirata , et al. |
October 19, 2021 |
Optical scanning device and method of adjusting optical scanning
device
Abstract
An optical scanning device includes a mirror part having a
mirror surface configured to reflect light, N support cantilevers
supporting the mirror part swingably, N drive cantilevers, and a
plurality of driving piezoelectric elements secured on N drive
cantilevers. The mirror part precesses by setting the frequency of
AC voltage applied to each of a plurality of piezoelectric elements
to a determined common value and setting the phase of AC voltage
applied to each of a plurality of piezoelectric elements to a value
determined according to the position of each piezoelectric
element.
Inventors: |
Hirata; Yoshiaki (Chiyoda-ku,
JP), Konno; Nobuaki (Chiyoda-ku, JP), Ito;
Takahiko (Chiyoda-ku, JP), Ishida; Kozo
(Chiyoda-ku, JP), Kajiyama; Yoshitaka (Chiyoda-ku,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Electric Corporation |
Chiyoda-ku |
N/A |
JP |
|
|
Assignee: |
MITSUBISHI ELECTRIC CORPORATION
(Tokyo, JP)
|
Family
ID: |
64660105 |
Appl.
No.: |
16/608,035 |
Filed: |
March 16, 2018 |
PCT
Filed: |
March 16, 2018 |
PCT No.: |
PCT/JP2018/010401 |
371(c)(1),(2),(4) Date: |
October 24, 2019 |
PCT
Pub. No.: |
WO2018/230065 |
PCT
Pub. Date: |
December 20, 2018 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200271920 A1 |
Aug 27, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
Jun 13, 2017 [JP] |
|
|
JP2017-115812 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S
7/4817 (20130101); G01S 7/4972 (20130101); G02B
26/0858 (20130101); G02B 26/101 (20130101); B81B
3/00 (20130101); G01S 7/481 (20130101); G02B
26/10 (20130101); G02B 26/08 (20130101) |
Current International
Class: |
G02B
26/08 (20060101); G02B 26/10 (20060101) |
Field of
Search: |
;359/200.8 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
106461934 |
|
Feb 2017 |
|
CN |
|
2007-206480 |
|
Aug 2007 |
|
JP |
|
2010-197994 |
|
Sep 2010 |
|
JP |
|
2011-100103 |
|
May 2011 |
|
JP |
|
2013-167681 |
|
Aug 2013 |
|
JP |
|
2014-92630 |
|
May 2014 |
|
JP |
|
2017-3717 |
|
Jan 2017 |
|
JP |
|
Other References
International Search Report dated Jun. 19, 2018 in
PCT/JP2018/010401 filed Mar. 16, 2018. cited by applicant .
Chinese Office Action dated May 31, 2021 in Chinese Application No.
201880030193.1. cited by applicant .
Chinese Office Action dated Aug. 24, 2021 in Chinese Application
No. 201880030193.1. cited by applicant.
|
Primary Examiner: Cherry; Euncha P
Attorney, Agent or Firm: Xsensus LLP
Claims
The invention claimed is:
1. An optical scanning device comprising: a mirror part having a
mirror surface configured to reflect light; N (N>3) support
cantilevers supporting the mirror part swingably; and N drive
cantilevers respectively connected to the N support cantilevers,
wherein the N drive cantilevers are arranged to surround the mirror
part, an end of both ends of each of the N drive cantilevers that
is not connected to the support cantilever is fixed, and each of
the N drive cantilevers has a shape bent one or more times, the
optical scanning device further comprising: a plurality of driving
piezoelectric elements secured on the N drive cantilevers; and a
power supply unit configured to apply AC voltage to the
piezoelectric elements, wherein the mirror part precesses by
setting a frequency of AC voltage applied to each of the
piezoelectric elements to a common value and setting a phase of AC
voltage applied to each of the piezoelectric elements to a value
determined according to a position of the piezoelectric element,
the N support cantilevers are arranged in (360.degree./N)
rotational symmetry with respect to a center axis of the mirror
part, the mirror part has a first natural frequency mode of being
rotationally displaced around a first axis and a second natural
frequency mode of being rotationally displaced around a second
axis, the first axis and the second axis being parallel to the
mirror surface, a direction of the first axis being a direction of
a straight line connecting a center of the mirror part with a
connection portion between the mirror part and one of the N support
cantilevers, the second axis being orthogonal to the first axis,
the power supply unit applies AC voltage at an intermediate
frequency between a resonance frequency in the first natural
frequency mode and a resonance frequency in the second natural
frequency mode, an amplitude and an initial phase of the AC voltage
are adjustable, and the power supply unit includes N drive power
supplies each applying AC voltage to a plurality of piezoelectric
elements on a corresponding drive cantilever, the optical scanning
device further comprising a control unit configured to, in
adjustment of the initial phase of the AC voltage, select one drive
cantilever from among the N drive cantilevers, fix the amplitude
and the initial phase of AC voltage applied to a plurality of
piezoelectric elements on one or more drive cantilevers other than
the selected drive cantilever, and fix the amplitude of AC voltage
applied to a plurality of piezoelectric elements on the selected
drive cantilever, and change the initial phase to determine the
initial phase at which rotation linearity error is smallest as an
adjustment value of the initial phase of AC voltage applied to a
plurality of piezoelectric elements on the selected drive
cantilever, wherein the rotation linearity error is a quantity that
represents a deviation from linearity of all-round rotational
displacement of the mirror part, and the all-round rotational
displacement of the mirror part is a displacement of the mirror
part such that the center axis of the mirror part makes a turn
while a deflection angle of the mirror part is kept constant.
2. An optical scanning device comprising: a mirror part having a
mirror surface configured to reflect light; N (N>3) support
cantilevers supporting the mirror part swingably; and N drive
cantilevers respectively connected to the N support cantilevers,
wherein the N drive cantilevers are arranged to surround the mirror
part, an end of both ends of each of the N drive cantilevers that
is not connected to the support cantilever is fixed, and each of
the N drive cantilevers has a shape bent one or more times, the
optical scanning device further comprising: a plurality of driving
piezoelectric elements secured on the N drive cantilevers: and a
power supply unit configured to apply AC voltage to the
piezoelectric elements, wherein the mirror part precesses by
setting a frequency of AC voltage applied to each of the
piezoelectric elements to a common value and setting a phase of AC
voltage applied to each of the piezoelectric elements to a value
determined according to a position of the piezoelectric element,
the N support cantilevers are arranged in (360.degree./N)
rotational symmetry with respect to a center axis of the mirror
part, the mirror part has a first natural frequency mode of being
rotationally displaced around a first axis and a second natural
frequency mode of being rotationally displaced around a second
axis, the first axis and the second axis being parallel to the
mirror surface, a direction of the first axis being a direction of
a straight line connecting a center of the mirror part with a
connection portion between the mirror part and one of the N support
cantilevers, the second axis being orthogonal to the first axis,
the power supply unit applies AC voltage at an intermediate
frequency between a resonance frequency in the first natural
frequency mode and a resonance frequency in the second natural
frequency mode, an amplitude and an initial phase of the AC voltage
are adjustable, and the power supply unit includes N drive power
supplies each applying AC voltage to a plurality of piezoelectric
of piezoelectric elements on a corresponding drive cantilever, the
optical scanning device further comprising a control unit
configured to, in adjustment of the amplitude of AC voltage, select
one drive cantilever from among the N drive cantilevers, fix the
amplitude and the initial phase of AC voltage applied to a
plurality of piezoelectric elements on one or more drive
cantilevers other than the selected drive cantilever, fix the
initial phase of AC voltage applied to a plurality of piezoelectric
elements on the selected drive cantilever, and change the amplitude
to determine the amplitude at which rotation linearity error is
smallest as an adjustment value of amplitude of AC voltage applied
to a plurality of piezoelectric elements on the selected drive
cantilever, wherein the rotation linearity error is a quantity that
represents a deviation from linearity of all-round rotational
displacement of the mirror part, and the all-round rotational
displacement of the mirror part is a displacement of the mirror
part such that the center axis of the mirror part makes a turn
while a deflection angle of the mirror part is kept constant.
3. An optical scanning device comprising: a mirror part having a
mirror surface configured to reflect light; N (N>3) support
cantilevers supporting the mirror part swingably; and N drive
cantilevers respectively connected to the N support cantilevers,
wherein the N drive cantilevers are arranged to surround the mirror
part, an end of both ends of each of the N drive cantilevers that
is not connected to the support cantilever is fixed, and each of
the N drive cantilevers has a shape bent one or more times, the
optical scanning device further comprising: a plurality of driving
piezoelectric elements secured on the N drive cantilevers: and a
power supply unit configured to apply AC voltage to the
piezoelectric elements, wherein the mirror part precesses by
setting a frequency of AC voltage applied to each of the
piezoelectric elements to a common value and setting a phase of AC
voltage applied to each of the piezoelectric elements to a value
determined according to a position of the piezoelectric element,
the N support cantilevers are arranged in (360.degree./N)
rotational symmetry with respect to a center axis of the mirror
part, the mirror part has a first natural frequency mode of being
rotationally displaced around a first axis and a second natural
frequency mode of being rotationally displaced around a second
axis, the first axis and the second axis being parallel to the
mirror surface, a direction of the first axis being a direction of
a straight line connecting a center of the mirror part with a
connection portion between the mirror part and one of the N support
cantilevers, the second axis being orthogonal to the first axis,
the power supply unit applies AC voltage at an intermediate
frequency between a resonance frequency in the first natural
frequency mode and a resonance frequency in the second natural
frequency mode, an amplitude and an initial phase of the AC voltage
are adjustable, wherein N=4, four drive cantilevers are ordered
clockwise or counterclockwise, four support cantilevers are
arranged in 90.degree. rotational symmetry with respect to the
center axis of the mirror part, four drive cantilevers are arranged
in 90.degree. rotational symmetry with respect to the center axis
of the mirror part, and, of portions that constitute each of the
four drive cantilevers, each of a plurality of circumferential
portions extending in a same direction as a circumferential
direction of the mirror part has two piezoelectric elements, the
power supply unit includes a first drive power supply configured to
output a first AC voltage having a first initial phase and a first
amplitude and a second AC voltage having a phase different from the
first AC voltage by 180 degrees, and a second drive power supply
configured to output a third AC voltage having a second initial
phase and a second amplitude and a fourth AC voltage having a phase
from the third AC voltage by 180 degrees, the first drive power
supply applies the first AC voltage to a first piezoelectric
element near the support cantilever, of a plurality of
piezoelectric elements arranged on the first drive cantilever, and
applies the first AC voltage or the second AC voltage to other
piezoelectric elements arranged on the first drive cantilever,
applies the second AC voltage to a second piezoelectric element
near the support cantilever, of a plurality of piezoelectric
elements arranged on the third drive cantilever, and applies the
first AC voltage or the second AC voltage to other piezoelectric
elements arranged on the third drive cantilever, and the second
drive power supply applies the third AC voltage to a third
piezoelectric element near the support cantilever, of a plurality
of piezoelectric elements arranged on the second drive cantilever,
applies the third AC voltage or the fourth AC voltage to other
piezoelectric elements arranged on the second drive cantilever,
applies the fourth AC voltage to a fourth piezoelectric element
near the support cantilever, of a plurality of piezoelectric
elements arranged on the fourth drive cantilever, and applies the
third AC voltage or the fourth AC voltage to other piezoelectric
elements arranged on the fourth drive cantilever.
4. The optical scanning device according to claim 3, further
comprising a control unit configured to fix the second initial
phase and the second amplitude and fix the first amplitude, change
the first initial phase to determine the first initial phase at
which rotation linearity error is smallest as an adjustment value
of the first initial phase, fix the second initial phase and the
second amplitude and fix the first initial phase, and change the
first amplitude to determine the first amplitude at which rotation
linearity error is smallest as an adjustment value of the first
amplitude, wherein the rotation linearity error is a quantity that
represents a deviation from linearity of all-round rotational
displacement of the mirror part, and the all-round rotational
displacement of the mirror part is a displacement of the mirror
part such that the center axis of the mirror part makes a turn
while a deflection angle of the mirror part is kept constant.
5. The optical scanning device according to claim 1, wherein the
mirror part has a trimming pattern on an outer periphery
thereof.
6. The optical scanning device according to claim 1, wherein the N
support cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to a center axis of the mirror part, and the
N drive cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to the center axis of the mirror part, the
mirror part has a (360.degree./N) rotational symmetric shape with
respect to the center axis of the mirror part, the mirror part is
formed using a crystal plane (100) of a semiconductor substrate,
and the N is 4.times.n, and the mirror surface has a circular
shape, and n is a natural number.
7. The optical scanning device according to claim 1, wherein the N
support cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to a center axis of the mirror part, and the
N drive cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to the center axis of the mirror part, the
mirror part has a (360.degree./N) rotational symmetric shape with
respect to the center axis of the mirror part, the mirror part is
formed using a crystal plane (100) of a semiconductor substrate,
and the N is 4.times.n, and the mirror surface has a regular
(4.times.n) polygonal shape, and n is a natural number.
8. The optical scanning device according to claim 1, wherein the N
support cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to a center axis of the mirror part, and the
N drive cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to the center axis of the mirror part, the
mirror part has a (360.degree./N) rotational symmetric shape with
respect to the center axis of the mirror part, the mirror part is
formed using a crystal plane (111) of a semiconductor substrate,
the N is 3.times.n, and the mirror surface has a circular shape,
and n is a natural number.
9. The optical scanning device according to claim 2, wherein the
mirror part has a trimming pattern on an outer periphery
thereof.
10. The optical scanning device according to claim 2, wherein the N
support cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to a center axis of the mirror part, and the
N drive cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to the center axis of the mirror part, the
mirror part has a (360.degree./N) rotational symmetric shape with
respect to the center axis of the mirror part, the mirror part is
formed using a crystal plane (100) of a semiconductor substrate,
and the N is 4.times.n, and the mirror surface has a circular
shape, and n is a natural number.
11. The optical scanning device according to claim 2, wherein the N
support cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to a center axis of the mirror part, and the
N drive cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to the center axis of the mirror part, the
mirror part has a (360.degree./N) rotational symmetric shape with
respect to the center axis of the mirror part, the mirror part is
formed using a crystal plane (100) of a semiconductor substrate,
and the N is 4.times.n, and the mirror surface has a regular
(4.times.n) polygonal shape, and n is a natural number.
12. The optical scanning device according to claim 2, wherein the N
support cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to a center axis of the mirror part, and the
N drive cantilevers are arranged in (360.degree./N) rotational
symmetry with respect to the center axis of the mirror part, the
mirror part has a (360.degree./N) rotational symmetric shape with
respect to the center axis of the mirror part, the mirror part is
formed using a crystal plane (111) of a semiconductor substrate,
the N is 3.times.n, and the mirror surface has a circular shape,
and n is a natural number.
13. The optical scanning device according to claim 1, further
comprising a control unit configured to, in adjustment of the
amplitude of AC voltage, select one drive cantilever from among the
N drive cantilevers, fix the amplitude and the initial phase of AC
voltage applied to a plurality of piezoelectric elements on one or
more drive cantilevers other than the selected drive cantilever,
fix the initial phase of AC voltage applied to a plurality of
piezoelectric elements on the selected drive cantilever, and change
the amplitude to determine the amplitude at which rotation
linearity error is smallest as an adjustment value of amplitude of
AC voltage applied to a plurality of piezoelectric elements on the
selected drive cantilever, wherein the rotation linearity error is
a quantity that represents a deviation from linearity of all-round
rotational displacement of the mirror part, and the all-round
rotational displacement of the mirror part is a displacement of the
mirror part such that the center axis of the mirror part makes a
turn while a deflection angle of the mirror part is kept
constant.
14. The optical scanning device according to claim 1, wherein N=4,
four drive cantilevers are ordered clockwise or counterclockwise,
four support cantilevers are arranged in 90.degree. rotational
symmetry with respect to the center axis of the mirror part, four
drive cantilevers are arranged in 90.degree. rotational symmetry
with respect to the center axis of the mirror part, and, of
portions that constitute each of the four drive cantilevers, each
of a plurality of circumferential portions extending in a same
direction as a circumferential direction of the mirror part has two
piezoelectric elements, the power supply unit includes a first
drive power supply configured to output a first AC voltage having a
first initial phase and a first amplitude and a second AC voltage
having a phase different from the first AC voltage by 180 degrees,
and a second drive power supply configured to output a third AC
voltage having a second initial phase and a second amplitude and a
fourth AC voltage having a phase from the third AC voltage by 180
degrees, the first drive power supply applies the first AC voltage
to a first piezoelectric element near the support cantilever, of a
plurality of piezoelectric elements arranged on the first drive
cantilever, and applies the first AC voltage or the second AC
voltage to other piezoelectric elements arranged on the first drive
cantilever, applies the second AC voltage to a second piezoelectric
element near the support cantilever, of a plurality of
piezoelectric elements arranged on the third drive cantilever, and
applies the first AC voltage or the second AC voltage to other
piezoelectric elements arranged on the third drive cantilever, and
the second drive power supply applies the third AC voltage to a
third piezoelectric element near the support cantilever, of a
plurality of piezoelectric elements arranged on the second drive
cantilever, applies the third AC voltage or the fourth AC voltage
to other piezoelectric elements arranged on the second drive
cantilever, applies the fourth AC voltage to a fourth piezoelectric
element near the support cantilever, of a plurality of
piezoelectric elements arranged on the fourth drive cantilever, and
applies the third AC voltage or the fourth AC voltage to other
piezoelectric elements arranged on the fourth drive cantilever.
15. The optical scanning device according to claim 2, wherein N=4,
four drive cantilevers are ordered clockwise or counterclockwise,
four support cantilevers are arranged in 90.degree. rotational
symmetry with respect to the center axis of the mirror part, four
drive cantilevers are arranged in 90.degree. rotational symmetry
with respect to the center axis of the mirror part, and, of
portions that constitute each of the four drive cantilevers, each
of a plurality of circumferential portions extending in a same
direction as a circumferential direction of the mirror part has two
piezoelectric elements, the power supply unit includes a first
drive power supply configured to output a first AC voltage having a
first initial phase and a first amplitude and a second AC voltage
having a phase different from the first AC voltage by 180 degrees,
and a second drive power supply configured to output a third AC
voltage having a second initial phase and a second amplitude and a
fourth AC voltage having a phase from the third AC voltage by 180
degrees, the first drive power supply applies the first AC voltage
to a first piezoelectric element near the support cantilever, of a
plurality of piezoelectric elements arranged on the first drive
cantilever, and applies the first AC voltage or the second AC
voltage to other piezoelectric elements arranged on the first drive
cantilever, applies the second AC voltage to a second piezoelectric
element near the support cantilever, of a plurality of
piezoelectric elements arranged on the third drive cantilever, and
applies the first AC voltage or the second AC voltage to other
piezoelectric elements arranged on the third drive cantilever, and
the second drive power supply applies the third AC voltage to a
third piezoelectric element near the support cantilever, of a
plurality of piezoelectric elements arranged on the second drive
cantilever, applies the third AC voltage or the fourth AC voltage
to other piezoelectric elements arranged on the second drive
cantilever, applies the fourth AC voltage to a fourth piezoelectric
element near the support cantilever, of a plurality of
piezoelectric elements arranged on the fourth drive cantilever, and
applies the third AC voltage or the fourth AC voltage to other
piezoelectric elements arranged on the fourth drive cantilever.
16. The optical scanning device according to claim 1, wherein the N
drive cantilevers are ordered clockwise or counterclockwise, a
phase of AC voltage applied by the power supply unit to a first
piezoelectric element on an i-th drive cantilever is larger than a
phase of voltage applied to a second piezoelectric element on an
(i-1)th drive cantilever by (360.degree./N), and a position of the
second piezoelectric element in the (i-1)th drive cantilever is
same as a position of the first piezoelectric element in the i-th
drive cantilever.
17. The optical scanning device according to claim 2, wherein the N
drive cantilevers are ordered clockwise or counterclockwise, a
phase of AC voltage applied by the power supply unit to a first
piezoelectric element on an i-th drive cantilever is larger than a
phase of voltage applied to a second piezoelectric element on an
(i-1)th drive cantilever by (360.degree./N), and a position of the
second piezoelectric element in the (i-1)th drive cantilever is
same as a position of the first piezoelectric element in the i-th
drive cantilever.
18. The optical scanning device according to claim 3, wherein the N
drive cantilevers are ordered clockwise or counterclockwise, a
phase of AC voltage applied by the power supply unit to a first
piezoelectric element on an i-th drive cantilever is larger than a
phase of voltage applied to a second piezoelectric element on an
(i-1)th drive cantilever by (360.degree./N), and a position of the
second piezoelectric element in the (i-1)th drive cantilever is
same as a position of the first piezoelectric element in the i-th
drive cantilever.
Description
TECHNICAL FIELD
The present invention relates to an optical scanning device and a
method of adjusting the same, more specifically to an optical
scanning device available for laser distance sensors or optical
scanners, for example, and a method of adjusting the same.
BACKGROUND ART
Microelectromechanical systems (MEMS) mirrors are known as optical
scanning devices for use in laser distance sensors or optical
scanners, for example. The mirror part of a MEMS mirror is driven
using electrostatic force, electromagnetic force, or piezoelectric
force.
When electrostatic force is used, the generated driving force is
small and a sufficient deflection angle is not ensured. When
electromagnetic force is used, it is necessary to arrange a
permanent magnet externally, which makes the device configuration
complicated and makes downsizing difficult.
When piezoelectric force is used, a piezoelectric element minutely
deformable is formed on a cantilever-like elastic member, and
in-plane distortion by piezoelectric force is converted into warp,
resulting in a large deformation. In the MEMS mirror described in
PTL 1, a piezoelectric element is arranged on a linear cantilever,
and a mirror on the linear cantilever axis is torsion-vibrated in a
single-axis or two-axis direction ([0099] to [00102] and FIG. 40 in
PTL 1).
The MEMS mirror described in PTL 2 includes mirror supports formed
at diagonal portions of a rectangular mirror, and a first actuator
and a second actuator arranged to surround the mirror part. Each of
the first actuator and the second actuator has a structure in which
a plurality of first piezoelectric cantilevers with a longitudinal
direction oriented in a first axis direction and a plurality of
second piezoelectric cantilevers with a longitudinal direction
oriented in a second axis direction are coupled in a folded manner.
One end of each actuator is connected to the mirror part through
the mirror support, and the other end is connected to a fixed
portion in the vicinity of the mirror support. Each actuator
rotationally vibrates in two axes, namely, around the X axis and
the Y axis ([0029] to [0067] and FIG. 1 to FIG. 12 in PTL 2).
The MEMS mirror described in PTL 3 includes an annular elastic
frame. On a surface of the frame, a piezoelectric body is provided
which is divided into four, is symmetric with respect to the
center, and includes electrodes having the same area. This MEMS
mirror includes a plurality of torsion bars extending in the radial
direction of the annular frame and a mirror panel connected to the
torsion bars. The MEMS mirror rotationally vibrates in plane while
keeping the deflection angle of the mirror optical axis to enable
two-dimensional scanning ([0009] to [0012] and FIG. 1 in PTL
3).
In any of the MEMS mirrors in PTLs 1 to 3, in order to convert a
minute deformation of piezoelectric force into a large deformation,
the piezoelectric element is driven at a resonance frequency of the
MEMS mirror.
CITATION LIST
Patent Literature
PTL 1: Japanese Patent Laying-Open No. 2010-197994
PTL 2: Japanese Patent Laying-Open No. 2013-167681
PTL 3: Japanese Patent Laying-Open No. 2007-206480
SUMMARY OF INVENTION
Technical Problem
A wide in-plane scanning range is required for a MEMS mirror used
in laser distance sensors and the like. The MEMS mirror capable of
all-round rotational displacement enables information acquisition
in in-plane 360-degree surroundings.
Unfortunately, since the conventional MEMS mirrors described in
PTLs 1 to 3 performs a scan by supporting a mirror part by a
silicon cantilever and distorting the cantilever, the in-plane scan
range is limited by fracture stress limit of silicon.
In view of the problems above, the present invention provides an
optical scanning device having a wide in-plane scan angle and a
method of adjusting the same.
Solution to Problem
According to an aspect of the present invention, an optical
scanning device includes a mirror part having a mirror surface
configured to reflect light, N (N.gtoreq.3) support cantilevers
supporting the mirror part swingably, and N drive cantilevers
respectively connected to the N support cantilevers. The N drive
cantilevers are arranged to surround the mirror part. An end of
both ends of each of the N drive cantilevers that is not connected
to the support cantilever is fixed. Each of the N drive cantilevers
has a shape bent one or more times. The optical scanning device
further includes a plurality of driving piezoelectric elements
secured on the N drive cantilevers and a power supply unit
configured to apply AC voltage to the piezoelectric elements. The
mirror part precesses by setting a frequency of AC voltage applied
to each of the piezoelectric elements to a common value and setting
a phase of AC voltage applied to each of the piezoelectric elements
to a value determined according to a position of the piezoelectric
element.
Advantageous Effects of Invention
According to the present invention, the mirror part precesses,
thereby increasing the in-plane scan range of the mirror part.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram illustrating a configuration and a front
surface of the main part of an optical scanning device in a first
embodiment.
FIG. 2 is a diagram illustrating a back surface of the main part of
the optical scanning device in first embodiment.
FIG. 3 is a diagram illustrating warp deformation of a drive
cantilever.
FIG. 4 is a diagram illustrating displacement of mirror part 1 in a
natural frequency mode 1.
FIG. 5 is a diagram illustrating displacement of mirror part 1 in a
natural frequency mode 2.
FIG. 6 is a diagram illustrating displacement of mirror part 1 in a
natural frequency mode 3.
FIG. 7 is a diagram illustrating a deflection angle of the mirror
part.
FIG. 8 is a diagram for explaining AC voltage applied to
piezoelectric elements on the drive cantilever in a second
embodiment.
FIG. 9 is a diagram illustrating temporal change of displacement of
the mirror part when AC voltage of Equation (1) is applied to some
of piezoelectric elements and AC voltage of Equation (2) is applied
to the remaining piezoelectric elements.
FIG. 10 is a diagram for explaining a first voltage application
method A.
FIG. 11 is a diagram for explaining a second voltage application
method B.
FIG. 12 is a diagram for explaining a third voltage application
method C.
FIG. 13 is a diagram illustrating the drive characteristics of the
mirror part in the first voltage application method A, the second
voltage application method B, and the third voltage application
method C.
FIG. 14 is a schematic diagram of a laser distance sensor including
the optical scanning device.
FIGS. 15(a) to 15(i) are cross-sectional views of the optical
scanning device in a manufacturing process.
FIG. 16 is a diagram for explaining AC voltage applied to
piezoelectric elements on the drive cantilever in a third
embodiment.
FIG. 17 is a diagram illustrating the front surface of the main
part of the optical scanning device in a fourth embodiment.
FIG. 18 is a diagram illustrating the front surface of the main
part of the optical scanning device in a fifth embodiment.
FIG. 19 is a diagram illustrating the front surface of the main
part of the optical scanning device in a sixth embodiment.
FIG. 20 is a diagram illustrating the front surface of the main
part of the optical scanning device in a seventh embodiment.
FIG. 21 is a diagram illustrating change in mirror largest
displacement inscribed angle .PSI.m against change in the applied
voltage phase .PHI. in displacement in three patterns.
FIG. 22 is a diagram illustrating the plot line B in FIG. 21 and an
approximate straight line of the plot line B.
FIG. 23 is a diagram illustrating the rotation linearity error of
the plot line B.
FIG. 24 is a diagram for explaining voltage for driving the
piezoelectric elements in the optical scanning device in an eighth
embodiment.
FIG. 25 is a diagram illustrating change in mirror largest
displacement inscribed angle .PSI.m against change in the applied
voltage phase .PHI. when AC voltage with resonance frequency F2 in
the natural frequency mode 2 is applied.
FIG. 26 is a diagram illustrating the relation between applied
voltage phase .PHI. and mirror largest displacement inscribed angle
.PSI.m when AC voltage with resonance frequency F3 in the natural
frequency mode 3 is applied.
FIG. 27 is a diagram illustrating the relation between applied
voltage phase .PHI. and mirror largest displacement inscribed angle
.PSI.m when AC voltage with an intermediate frequency between
resonance frequency F2 in the natural frequency mode 2 and
resonance frequency F3 in the natural frequency mode 3 is
applied.
FIG. 28 is a diagram illustrating the relation between the
frequency of drive voltage and the rotation linearity error.
FIG. 29 is a flowchart illustrating the procedure for adjusting
voltages of drive power supplies for allowing the mirror part to
make all-round rotational displacement with the intermediate
frequency Fm between the natural frequency mode 2 and the natural
frequency mode 3 in the optical scanning device in the eighth
embodiment.
FIGS. 30(a) to 30(d) are diagrams illustrating the relation between
initial phase .PHI.1, .PHI.2, .PHI.3, .PHI.4 and the measured
rotation linearity error, and FIGS. 30(e) to 30(h) are diagrams
illustrating the relation between the amplitude Vs1, Vs2, Vs3, Vs4
and the measured rotation linearity error.
FIG. 31 is a diagram illustrating the applied voltage phase .PHI.
and the measured mirror largest displacement inscribed angle .PSI.m
after the adjustment of amplitude and initial phase of output
voltage of a drive power supply.
FIG. 32 is a flowchart illustrating the procedure for adjusting
output voltages of the drive power supplies for allowing the mirror
part to make all-round rotational displacement with the
intermediate frequency Fm between the natural frequency mode 2 and
the natural frequency mode 3 in the optical scanning device in a
ninth embodiment.
FIG. 33 is a diagram for explaining voltage for driving
piezoelectric elements in the optical scanning device in a tenth
embodiment.
FIG. 34 is a flowchart illustrating the procedure for adjusting
voltages of the drive power supplies for allowing the mirror part
to make all-round rotational displacement with the intermediate
frequency Fm between the natural frequency mode 2 and the natural
frequency mode 3 in the optical scanning device in the tenth
embodiment.
FIG. 35(a) is a diagram illustrating the relation between the
initial phase .PHI.1 and the measured rotation linearity error, and
FIG. 35(b) is a diagram illustrating the relation between the
amplitude Vs1 and the measured rotation linearity error.
FIG. 36 is a diagram illustrating the mirror part of the optical
scanning device in an eleventh embodiment.
FIG. 37 is an enlarged view of a trimming pattern of the mirror
part.
FIG. 38 is a diagram illustrating the relation between the drive
frequency and the mirror largest displacement before trimming.
FIG. 39 is a diagram illustrating the relation between the drive
frequency and the mirror largest displacement after trimming.
FIG. 40 is a flowchart illustrating the procedure for adjusting the
optical scanning device in the eleventh embodiment.
FIG. 41 is a flowchart illustrating the adjustment procedure for
the optical scanning device in a twelfth embodiment.
FIG. 42 is a diagram illustrating an example of the optical
scanning device in a thirteenth embodiment.
FIG. 43 is a diagram illustrating another example of the optical
scanning device in the thirteenth embodiment.
FIG. 44 is a flowchart illustrating the procedure for adjusting the
optical scanning device in the thirteenth embodiment.
DESCRIPTION OF EMBODIMENTS
Embodiments of the present invention will be described below with
reference to the drawings.
First Embodiment
FIG. 1 is a diagram illustrating a configuration of an optical
scanning device 100 in a first embodiment and a front surface of
the main part of optical scanning device 100. FIG. 2 is a diagram
illustrating a back surface of the main part of optical scanning
device 100 in the first embodiment.
Referring to FIG. 1 and FIG. 2, optical scanning device 100
includes a mirror part 1, drive cantilevers 3-1, 3-2, 3-3, 3-4,
support cantilevers 2-1, 2-2, 2-3, 2-4, piezoelectric elements
5-1-a, b, c, d, piezoelectric elements 5-2-a, b, c, d,
piezoelectric elements 5-3-a, b, c, d, piezoelectric elements
5-4-a, b, c, d, and detecting piezoelectric elements 6-1, 6-2, 6-3,
6-4.
Mirror part 1 is a MEMS mirror. Mirror part 1 includes a mirror
surface 1B and a silicon mirror part 1C. Mirror surface 1B receives
light from a light source and reflects light. The front surface
(mirror surface 1B) and the back surface of mirror part 1 are
circular. Mirror surface 1B is made of a metal thin film with high
reflectivity. Silicon mirror part 1C is obtained by processing a
silicon substrate.
When mirror part 1 is at rest, the normal direction of mirror part
1 is the Z axis. Here, the center axis of mirror part 1 matches the
Z axis. When mirror part 1 is at rest, two orthogonal axes parallel
to the surfaces (one of which is mirror surface 1B) of mirror part
1 are the X axis and the Y axis. The direction of the X axis is the
direction from the center O of mirror part 1 to support cantilever
2-1. The direction of the Y axis is the direction from the center O
of mirror part 1 to support cantilever 2-2. The Z axis also serves
as the center axis and the optical axis of mirror part 1 at
rest.
A mirror inscribed angle .PSI. at a certain point on the
circumference of mirror part 1 is the angle between the line
connecting the point with the center O of mirror part 1 and the X
axis.
Support cantilevers 2-1 to 2-4 support silicon mirror part 1C
swingably. Support cantilevers 2-1 to 2-4 have the same shape and
size. Support cantilevers 2-1 to 2-4 are arranged in 90.degree.
rotational symmetry with respect to the center axis (the Z axis) of
mirror part 1.
Drive cantilevers 3-1 to 3-4 are arranged to surround mirror part
1. Drive cantilevers 3-1 to 3-4 have the same shape and size. One
end of drive cantilever 3-i (i=1 to 4) is connected to support
cantilever 2-i and the other end of drive cantilever 3-i is
connected to a fixed portion 4-i. Drive cantilevers 3-1 to 3-4 are
arranged in 90.degree. rotational symmetry with respect to the
center axis of mirror part 1. Drive cantilevers 3-1 to 3-4 each
have a shape bent once at 180.degree.. Drive cantilevers 3-1 to 3-4
each have two circumferential portions extending in the same
direction as the circumferential direction of mirror part 1 and a
bend portion. In other words, drive cantilevers 3-1 to 3-4 are each
U-shaped. However, the portion in the longitudinal direction of the
U shape is curved in the same direction as the circumference
direction of mirror part 1.
Support cantilevers 2-1 to 2-4 are ordered counterclockwise.
Support cantilever 2-i is the i-th support cantilever. Drive
cantilevers 3-1 to 3-4 are ordered counterclockwise. Drive
cantilever 3-i is the i-th drive cantilever. Alternatively, support
cantilevers 2-1 to 2-4 and drive cantilevers 3-1 to 3-4 may be
ordered clockwise, instead. That is, support cantilevers 2-1, 2-2,
2-3, 2-4 may be the first, fourth, third, and second support
cantilevers. Drive cantilevers 3-1, 3-2, 3-3, 3-4 may be the first,
fourth, third, and second drive cantilevers.
Piezoelectric elements 5-i-a, b, c, d (i=1 to 4) are secured on
drive cantilever 3-i. Sixteen piezoelectric elements 5-i-a, b, c, d
have the same shape and size.
Of the portions that constitute drive cantilever 3-i (i=1 to 4), a
first portion of two circumferential portions extending in the same
direction as the circumferential direction of mirror part 1 has
piezoelectric elements 5-i-a, b, and a second portion has
piezoelectric elements 5-i-c, d. The first portion is positioned
closer to mirror part 1 than the second portion. Piezoelectric
element 5-i-a and piezoelectric element 5-i-b are spaced apart from
each other. Piezoelectric element 5-i-c and piezoelectric element
5i-d are spaced apart from each other. Piezoelectric element 5-i-b
and piezoelectric element 5i-c are adjacent to each other with the
bend portion of drive cantilever 3-i interposed. Piezoelectric
element 5i-a is arranged at a position closest to one end of drive
cantilever 3-i connected to support cantilever 2-i. Piezoelectric
element 5i-d is arranged at a position closest to the other end of
drive cantilever 3-i connected to fixed portion 4-i.
Although not illustrated in the figures, piezoelectric element
5i-a, b, c, d (i=1 to 4) is connected to an upper electrode and a
lower electrode for applying a drive voltage. In the present
embodiment, the lower electrodes of piezoelectric elements 5-i-a,
b, c, d are grounded, and a drive voltage is applied to the upper
electrodes of piezoelectric elements 5i-a, b, c, d, so that
dielectric polarization directions of all piezoelectric elements
5i-a, b, c, d are identical. Alternatively, the upper electrodes of
piezoelectric elements 5i-a, b, c, d may be grounded, and voltage
may be applied to the lower electrodes of piezoelectric elements
5i-a, b, c, d, so that the dielectric polarization directions of
all piezoelectric elements 5i-a, b, c, d are identical.
A power supply unit 62 applies AC voltage to piezoelectric elements
5i-a, b, c, d. A control unit 61 controls output voltage of power
supply unit 62.
When power supply unit 62 applies a drive voltage to piezoelectric
elements 5-i-a, b, c, d (i=1 to 4), piezoelectric elements 5i-a, b,
c, d expand and contract on a plane parallel to the XY plane so
that drive cantilever 3-i warps as a whole. FIG. 3 is a diagram
illustrating an example of warp deformation of drive cantilever
3-i. Drive cantilevers 3-2 to 3-4 are deformed similarly. Because
of the warp deformation of drive cantilever 3-i, the connection
portion between drive cantilever 3-i and support cantilever 2-i is
driven in the Z axis direction, whereby mirror part 1 is
driven.
Mirror part 1 precesses by setting the frequency of AC voltage
applied to each of a plurality of piezoelectric elements 5i-a, b,
c, d to a predetermined common value and setting the phase of AC
voltage applied to each of a plurality of piezoelectric elements
5i-a, b, c, d to a value determined according to the position of
the piezoelectric element.
That is, with application of such AC voltage to a plurality of
piezoelectric elements, the mirror inscribed angle .PSI. at which
the out-of-plane direction of the outer peripheral portion of
mirror part 1 achieves the largest displacement changes with time
at regular intervals, whereby mirror part 1 precesses.
Silicon mirror part 1C is formed of a support layer and an active
layer of a silicon on insulator (SOI) substrate. Support
cantilevers 2-1 to 2-4 and drive cantilevers 3-1 to 3-4 are formed
of a support layer of a SOI substrate.
As described above, according to the first embodiment, since mirror
part 1 precesses, the in-plane scan range of mirror part 1 is
increased.
(Note)
Optical scanning device 100 in the first embodiment has the
following features.
(1) Optical scanning device 100 includes a mirror part (1) having a
mirror surface (1B) configured to reflect light, N (N.gtoreq.3)
support cantilevers (2-1 to 2-4) supporting the mirror part (1)
swingably, and N drive cantilevers (3-1 to 3-4) respectively
connected to N support cantilevers (2-1 to 2-4). N drive
cantilevers (3-1 to 3-4) are arranged to surround mirror part 1. An
end of both ends of each of N drive cantilevers (3-1 to 3-4) that
is not connected to the support cantilever (2-1 to 2-4) is fixed.
Each of N drive cantilevers (3-1 to 3-4) has a shape bent one or
more times.
Optical scanning device 100 further includes a plurality of driving
piezoelectric elements (5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a
to d) secured on N drive cantilevers (3-1 to 3-4) and a power
supply unit (62) configured to apply AC voltage to a plurality of
piezoelectric elements (5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a
to d). The mirror part (1) precesses by setting the frequency of AC
voltage applied to each of a plurality of piezoelectric elements
(5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a to d) to a predetermined
command value and setting the phase of AC voltage applied to each
of a plurality of piezoelectric elements 5-1-a to d, 5-2-a to d,
5-3-a to d, 5-4-a to d) to a predetermined value according to the
position of the piezoelectric element.
Such a configuration allows the mirror part to precess and thereby
increases the scan range of the mirror part.
Second Embodiment
The result of modal analysis of mirror part 1 is described.
FIG. 4 is a diagram illustrating displacement of mirror part 1 in a
natural frequency mode 1. FIG. 5 is a diagram illustrating
displacement of mirror part 1 in a natural frequency mode 2. FIG. 6
is a diagram illustrating displacement of mirror part 1 in a
natural frequency mode 3.
Referring to FIG. 4 to FIG. 6, in natural frequency mode 1, mirror
part 1 is translationally displaced in the Z axis direction. In
natural frequency mode 2, mirror part 1 is rotationally displaced
around a first axis. In natural frequency mode 3, mirror part 1 is
rotationally displaced around a second axis.
The first axis is parallel to mirror surface 1B, and the direction
of the first axis is the direction of a straight line connecting
the center of mirror part 1 with the connection portion between
mirror part 1 and support cantilever 2-1. The second axis is
parallel to mirror surface 1B. The direction of the second axis is
the direction of a straight line connecting the center of mirror
part 1 with the connection portion between mirror part 1 and
support cantilever 2-2. In FIG. 4, the first axis is the X axis,
and the second axis is the Y axis.
When the optical scanning device includes N support cantilevers
arranged in (360.degree./N) rotational symmetry with respect to the
center axis of mirror part 1, the configuration is as follows.
The first axis is parallel to mirror surface 1B. The direction of
the first axis is the direction of a straight line connecting the
center of mirror part 1 with the connection portion between mirror
part 1 and one of N support cantilevers. The second axis is
parallel to mirror surface 1B and orthogonal to the first axis.
When the shape of mirror part 1 has 90-degree rotational symmetry
on the XY plane, the resonance frequency in the natural frequency
mode 2 matches the resonance frequency in the natural frequency
mode 3. It is assumed that the resonance frequency in the natural
frequency mode 2 and the natural frequency mode 3 when the shape of
mirror part 1 has 90-degree rotational symmetry on the XY plane is
F0.
Power supply unit 62 applies AC voltage having the resonance
frequency F0 as a frequency to piezoelectric elements 5i-a, b, c, d
(i=1 to 4), whereby a small strain of piezoelectric elements 5i-a,
b, c, d causes a large displacement of drive cantilever 3-i (i=1 to
4). This can deflect mirror part 1 in the Z axis direction.
FIG. 7 is a diagram illustrating a deflection angle .theta. of
mirror part 1.
As shown in FIG. 7, the deflection angle .theta. of mirror part 1
is the angle between mirror part 1 and the XY plane.
FIG. 8 is a diagram for explaining AC voltage applied to
piezoelectric elements 5i-a to d on drive cantilever 3-i (i=1 to 4)
in a second embodiment.
Power supply unit 62 applies AC voltage of Equation (1) to
piezoelectric elements 5i-a, 5i-c on drive cantilever 3-i (i=1 to
4), where i=1 to 4.
Vi(1)=Vs.times.sin(.omega.t+90.degree..times.(i-1)) (1)
Here, .omega.=2.pi..times.F0. F0 is the resonance frequency in the
natural frequency mode 2 and the resonance frequency in the natural
frequency mode 3. t is time.
Power supply unit 62 applies AC voltage of Equation (2) to
piezoelectric elements 5i-b, 5i-d on drive cantilever 3-i (i=1 to
4), where i=1 to 4.
Vi(2)=-Vi(1)=-Vs.times.sin(.omega.t+90.degree..times.(i-1)) (2)
Since the signs of Vi(1) in Equation (1) and Vi(2) in Equation (2)
are reversed, the phase of Vi(1) in Equation (1) and the phase of
Vi(2) in Equation (2) differ by 180.degree..
FIG. 9 is a diagram illustrating temporal change of displacement of
mirror part 1 when AC voltage of Equation (1) is applied to
piezoelectric elements 5i-a, c (i=1 to 4) and AC voltage of
Equation (2) is applied to piezoelectric elements 5i-b, d. As shown
in FIG. 9, as the time progresses
(t1.fwdarw.t2.fwdarw.t3.fwdarw.t4), the portion of mirror part 1
that deflects in the Z axis direction turns around the Z axis. Such
displacement is called all-round rotational displacement of mirror
part 1. In other words, all-round rotational displacement of mirror
part 1 refers to displacement of mirror part 1 in such a manner
that the center axis of mirror part 1 makes a turn while the
deflection angle .theta. of mirror part 1 is kept constant.
Referring to FIG. 1 again, detecting piezoelectric element 6-i is
arranged at the joint portion between support cantilever 2-i and
drive cantilever 3-i. The reason why it is arranged at the joint
portion is that the joint portion has high stress. Detecting
piezoelectric element 6-i is connected with an upper electrode and
a lower electrode, although not shown in the figures. Detecting
piezoelectric element 6-i generates electric charge proportional to
the deflection angle .theta. of mirror part 1.
Control unit 61 monitors the electric charge generated in detecting
piezoelectric element 6-i to measure the deflection angle .theta.
of mirror part 1. Control unit 61 can control the deflection angle
.theta. by adjusting the value of amplitude Vs in Equations (1) and
(2).
The difference in drive characteristics of mirror part 1 between a
first voltage application method A in the present embodiment above
and other second and third voltage application methods B and C will
now be described.
FIG. 10 is a diagram for explaining the first voltage application
method A. In the first voltage application method A, voltage of
Equation (1) is applied to piezoelectric element 5i-a and
piezoelectric element 5i-c, and voltage of Equation (2) is applied
to piezoelectric element 5i-b and piezoelectric element 5i-d.
FIG. 11 is a diagram for explaining the second voltage application
method B. In the second voltage application method B, voltage of
Equation (1) is applied to piezoelectric element 5i-a and
piezoelectric element 5i-b, and voltage of Equation (2) is applied
to piezoelectric element 5i-c and piezoelectric element 5i-d.
FIG. 12 is a diagram for explaining the third voltage application
method C. In the third voltage application method C, voltage of
Equation (1) is applied to piezoelectric element 5i-a and
piezoelectric element 5i-d, and voltage of Equation (2) is applied
to piezoelectric element 5i-b and piezoelectric element 5i-c.
FIG. 13 is a diagram illustrating the drive characteristics of
mirror part 1 in the first voltage application method A, the second
voltage application method B, and the third voltage application
method C.
The vertical axis in FIG. 13 shows the deflection angle .theta. of
mirror part 1. As shown in FIG. 13, the deflection angle .theta. is
largest in the first voltage application method A.
FIG. 14 is a schematic diagram of a laser distance sensor including
optical scanning device 100.
The laser distance sensor includes a sensing unit 99, an optical
system 91, and optical scanning device 100. Sensing unit 99
includes a distance information calculator 93, a drive circuit 96,
a laser diode LD, a photodiode PD, and a receiver circuit 95.
Distance information calculator 93 instructs drive circuit 96 to
output emission light in order to calculate the distance to a
target 92.
Drive circuit 96 drives laser diode LD. Transmission light emitted
by laser diode LD is collected by optical system 91 and applied to
mirror part 1. Light reflected by mirror part 1 is reflected by
optical system 91 and sent to target 92.
Light scattered by target 92 is directed again to mirror part 1.
Light reflected by mirror part 1 is collected by optical system 91
and sent to photodiode PD. Photodiode PD converts the detected
received light into voltage and sends the voltage to receiver
circuit 95. Receiver circuit 95 notifies distance information
calculator 93 that received light is input, based on change in
voltage from photodiode PD.
Distance information calculator 93 calculates the distance to
target 92 based on the difference between the time at which
emission light is output and the time at which received light is
input.
A method of manufacturing optical scanning device 100 will now be
described.
FIGS. 15(a) to 15(i) are cross-sectional views of optical scanning
device 100 in a manufacturing process.
Mirror part 1 and the like are formed using the crystal plane (100)
of an SOI substrate which is a kind of semiconductor
substrates.
In FIG. 15(a), an SOI substrate 10 is used, in which a silicon
oxide film 103 is sandwiched between a monocrystalline silicon
support layer 101 and a monocrystalline silicon active layer 102.
The mechanical property of the crystal plane (100) of the SOI
substrate is 4-fold symmetry.
In FIG. 15(b), an insulating film 11 is formed on the front surface
and the back surface of SOI substrate 10, for example, by thermal
oxidation.
In FIG. 15(c), a lower layer electrode 12 such as Ti/Pt is formed
on the front surface side of SOI substrate 10, for example, by
sputtering, and a piezoelectric thin film 13 such as lead zirconate
titanate (PZT) is formed thereon, for example, by sputtering or
sol-gel process, and an upper layer electrode 14 such as Ti/Pt is
formed thereon, for example, by sputtering.
In FIG. 15(d), lower layer electrode 12, piezoelectric thin film
13, and upper layer electrode 14 are patterned. Piezoelectric thin
film 13 serves as driving piezoelectric elements 5i-a to d (i=1 to
4) and detecting piezoelectric element 6-i of mirror part 1. Lower
layer electrode 12 is used not only for the lower layer electrode
of piezoelectric thin film 13 but also for wiring. Lower layer
electrode 12 on the left side in FIG. 15(d) is a wiring
electrode.
In FIG. 15(e), an insulating film 15 such as an oxide film is
formed and patterned. Insulating film 15 is a film for protecting
piezoelectric thin film 13 and preventing short-circuit between
electrodes, and a contact hole between lower layer electrode 12 and
upper layer electrode 14 is formed for leading out an
electrode.
In FIG. 15(f), a wiring electrode 16 and a mirror surface 1B are
formed. Wiring electrode 16 is a low-resistance metal such as
Ti/Pd/Au, and the surface of mirror surface 1B is a
high-reflectivity material such as Au. Wiring electrode 16 and
mirror surface 1B may be the same material. Mirror surface 1B may
have a high-reflectivity dielectric multilayer film on its
surface.
In FIG. 15(g), silicon active layer 102 is processed using an oxide
film 103 as an etching stop layer by deep reactive ion etching
(DRIE). In this etching, main constituent parts of optical scanning
device 100, such as drive cantilevers 3-1 to 3-4 and support
cantilevers 2-1 to 2-4, are formed.
In FIG. 15(h), silicon support layer 101 is processed from its back
surface by DRIE to remove unnecessary parts of silicon oxide film
103. In this process, mirror part 1 and the like are formed, and
the wafer process of the optical scanning device is completed.
In FIG. 15(i), a chip is isolated and then bonded to a package 17
and electrically connected to the outside, for example, by wire
bonding. Package 17 may be sealed under reduced pressure atmosphere
in order to increase the deflection angle .theta. as necessary. The
method of manufacturing an optical scanning device in the present
embodiment does not include a special process and therefore can be
carried out using a typical silicon MEMS fabrication facility and a
piezoelectric element fabrication facility.
As described above, in the second embodiment, four drive
cantilevers are driven with a 90-degree phase difference by the
rotational displacement resonance frequency around the support
cantilever, whereby mirror part 1 can make all-round rotational
displacement with a large deflection angle (inclination angle).
The structure of the drive cantilever extending along the outer
periphery can reduce the dedicated area for the drive cantilever.
The U-shaped bend shape of the drive cantilever can reduce stress
exerted on the drive cantilever and stress exerted on the
piezoelectric element serving as a drive source, compared with a
drive cantilever with no bend structure, if the amount of
displacement of mirror part 1 is the same. Consequently, the
deflection angle can be increased. In addition, the U-shape of the
drive cantilever can reduce the drive cantilever spring constant in
the mirror in-plane direction. This can reduce misalignment of the
mirror surface and change in the deflection angle even when the
position of the fixed portion of the drive cantilever is shifted
due to temperature change or the like.
Since the deflection angle .theta. is affected by the viscosity of
air, the entire optical scanning device may be seal-packaged under
reduced pressure atmosphere in accordance with the required
deflection angle .theta..
In a conventional optical scanning device, because of limitations
of the in-plane scan range, the surroundings of the optical
scanning device are measured only partially.
However, in the optical scanning device in the present embodiment,
a combination of all-round rotational displacement and scanning at
the deflection angle enables acquisition of distance information in
the 360-degree surroundings around the Z axis. For example, by
scanning at a deflection angle .theta. of 10.+-.5 degrees, light
incident on mirror surface 1B can be received with the deflection
angle .theta. of 10.+-.5 degrees in the 360-degree surroundings
around the Z axis.
(Note)
Optical scanning device 100 in the second embodiment has the
following features.
(2) Optical scanning device 100 includes a mirror part (1) having a
mirror surface (1B) configured to reflect light, N (N.gtoreq.3)
support cantilevers (2-1 to 2-4) supporting the mirror part (1)
swingably, and N drive cantilevers (3-1 to 3-4) respectively
connected to N support cantilevers (2-1 to 2-4). N drive
cantilevers (3-1 to 3-4) are arranged to surround mirror part 1. An
end of both ends of each of N drive cantilevers (3-1 to 3-4) that
is not connected to the support cantilever (2-1 to 2-4) is fixed.
Each of N drive cantilevers (3-1 to 3-4) has a shape bent one or
more times. Optical scanning device 100 further includes a
plurality of driving piezoelectric elements (5-1-a to d, 5-2-a to
d, 5-3-a to d, 5-4-a to d) secured on N drive cantilevers (3-1 to
3-4) and a power supply unit (62) configured to apply AC voltage to
a plurality of piezoelectric elements (5-1-a to d, 5-2-a to d,
5-3-a to d, 5-4-a to d). N support cantilevers (2-1 to 2-4) are
arranged in (360.degree./N) rotational symmetry with respect the
center axis of the mirror part (1). The mirror part (1) has a first
natural frequency mode (natural frequency mode 2) of being
rotationally displaced around a first axis (the X axis) and a
second natural frequency mode (natural frequency mode 3) of being
rotationally displaced around a second axis (the Y axis). The first
axis (the X axis) and the second axis (the Y axis) are parallel to
the mirror surface (1B). The direction of the first axis (the X
axis) is the direction of a straight line connecting the center of
the mirror part (1) with a connection portion between the mirror
part (1) and one of N support cantilevers (2-1 to 2-4). The second
axis (the Y axis) is orthogonal to the first axis (the X axis).
Both of the resonance frequency in the first natural frequency mode
(natural frequency mode 2) and the resonance frequency in the
second natural frequency mode (natural frequency mode 3) are a
first frequency (F0). The power supply unit (62) applies AC voltage
at the first frequency (F0).
Such a configuration enables all-round rotational displacement of
the mirror part (1) with a large deflection angle.
(3) Two piezoelectric elements (5i-a and 5i-b or 5i-c and 5i-d) are
arranged on each of a plurality of circumferential portions
extending in the same direction as the circumferential direction of
the mirror part (1), of the portions that constitute each of N
drive cantilevers (3-i).
This configuration can minimize the number of piezoelectric
elements and also minimize the number of spacings between adjacent
piezoelectric elements. As a result, the area of the piezoelectric
elements that can be secured on the drive cantilever can be
increased and therefore the drive force of the mirror part (1) can
be increased.
(4) The dielectric polarization directions of a plurality of
piezoelectric elements (5i-a to d) arranged on each of N drive
cantilevers (3-i) are the same. The power supply unit (62) applies
voltages with opposite phases to two piezoelectric elements (5-i-a
and 5i-b, and 5i-c and 5i-d) arranged on each of a plurality of
circumferential portions, and applies AC voltages with phases
opposite to each other to two piezoelectric elements (5i-b and
5i-c) adjacent with the circumferential portion interposed
therebetween.
With such a configuration, AC voltage can be applied to a plurality
of piezoelectric elements (5i-a to d) such that the mirror part (1)
can make all-round rotational displacement.
(5) The control unit (61) controls the amplitude of AC voltage
output from the power supply unit (62) to scan the deflection angle
(.theta.) of the mirror part (1).
With such a configuration, three-dimensional information of a
target can be acquired.
(6) The mirror part (1) is formed using the crystal plane (100) of
a semiconductor substrate. N is 4.times.n (n is a natural number).
The shape of the mirror surface (1B) is circular.
With such a configuration, the mechanical properties of a plurality
of drive cantilevers can be made uniform.
Third Embodiment
FIG. 16 is a diagram for explaining AC voltage applied to
piezoelectric elements 5i-a to d on drive cantilever 3-i (i=1 to 4)
of an optical scanning device 200 in a third embodiment.
Optical scanning device 200 in the third embodiment differs from
optical scanning device 100 in the second embodiment in the
following points.
In the second embodiment, the lower electrodes of piezoelectric
elements 5i-a, b, c, d (i=1 to 4) are grounded, and voltage is
applied to the upper electrodes of piezoelectric elements 5i-a, b,
c, d, so that the dielectric polarization directions of all of 16
piezoelectric elements 5i-a, b, c, d are the same.
By contrast, in the present embodiment, the lower electrodes of
piezoelectric elements 5i-a, c (i=1 to 4) are grounded, and voltage
is applied to the upper electrodes of piezoelectric elements 5i-a,
c, so that the dielectric polarization direction of eight
piezoelectric elements 5i-a, c is a first direction, whereas the
upper electrodes of piezoelectric elements 5i-b, d are grounded,
and voltage is applied to the lower electrodes of piezoelectric
elements 5i-b, d, so that the dielectric polarization direction of
eight piezoelectric elements 5i-b, d is a second direction opposite
to the first direction.
In the second embodiment, power supply unit 62 applies AC voltage
of Equation (1) to eight piezoelectric elements 5i-a, 5i-c on drive
cantilevers 3-i (i=1 to 4) and applies AC voltage of Equation (2)
to eight piezoelectric elements 5i-b, 5i-d on drive cantilevers
3-i.
By contrast, in the third embodiment, power supply unit 62 applies
AC voltage of Equation (1) to 16 piezoelectric elements 5i-a, 5i-b,
5i-c, 5i-c on drive cantilever 3-i (i=1 to 4).
The third embodiment achieves the similar effects as in the first
embodiment.
(Note)
Optical scanning device 200 in the third embodiment has the
following features.
(7) The dielectric polarization directions of two piezoelectric
elements arranged on each of a plurality of circumferential
portions (5i-a and 5i-b, and 5i-c and 5i-d) are opposite. The
dielectric polarization directions of two piezoelectric elements
(5i-b and 5i-c) adjacent with the bend portion interposed
therebetween are opposite. The power supply unit (62) applies AC
voltage of the same phase to a plurality of piezoelectric elements
(5i-a to d) arranged on each of N drive cantilevers (3-i).
With such a configuration, even when the dielectric polarization
directions of adjacent piezoelectric elements are opposite
directions, a plurality of piezoelectric elements (5i-a to d) can
be driven such that the mirror part (1) can make all-round
rotational displacement.
Fourth Embodiment
FIG. 17 is a diagram illustrating the front surface of the main
part of an optical scanning device 300 in a fourth embodiment.
Optical scanning device 400 in the fourth embodiment differs from
optical scanning device 100 in the second embodiment in the
following points.
Referring to FIG. 17, drive cantilevers 3-1 to 3-4 are arranged to
surround mirror part 1. Drive cantilevers 3-1 to 3-4 have the same
shape and size. Drive cantilevers 3-1 to 3-4 are arranged in
90.degree. rotational symmetry with respect to the center axis of
mirror part 1.
One end of drive cantilever 3-i (i=1 to 4) is connected to support
cantilever 2-i and the other end of drive cantilever 3-i is
connected to fixed portion 4-i. Drive cantilever 3-i has a shape
bent twice at 180.degree..
Drive cantilever 3-i (i=1 to 4) has three circumferential portions
extending in the same direction as the circumferential direction of
mirror part 1 and bend portions. In other words, drive cantilevers
3-1 to 3-4 have a continuous U shape such that U-shaped portions
are formed serpentinely so as to be oriented in opposite directions
alternately. The longitudinal portion of the U shape is curved in
the same direction as the circumference direction of mirror part
1.
Piezoelectric elements 5i-a, b, c, d, e, f (i=1 to 4) are secured
on drive cantilever 3-i. Piezoelectric elements 5i-a, b, c, d, e, f
have the same shape and size.
Of the portions that constitute drive cantilever 3-i (i=1 to 4), a
first portion of three circumferential portions extending in the
same direction as the circumferential direction of mirror part 1
has piezoelectric elements 5i-a, b, a second portion has
piezoelectric elements 5i-c, d, and a third portion has
piezoelectric elements 5i-e, f. The first portion is at a position
closest to mirror part 1, and the third portion is at a position
furthest from mirror part 1.
Piezoelectric element 5i-a (i=1 to 4) and piezoelectric element
5i-b are spaced apart from each other. Piezoelectric element 5i-c
and piezoelectric element 5-i-d are spaced apart from each other.
Piezoelectric element 5i-e and piezoelectric element 5i-f are
spaced apart from each other. Piezoelectric element 5i-b and
piezoelectric element 5i-c are adjacent to each other with a first
bend portion of drive cantilever 3-i interposed therebetween.
Piezoelectric element 5i-d and piezoelectric element 5i-e are
adjacent to each other with a second bend portion of drive
cantilever 3-i interposed therebetween. Piezoelectric element 5i-a
is arranged at a position closest to one end of drive cantilever
3-i connected to support cantilever 2-i. Piezoelectric element 5i-f
is arranged at a position closest to the other end of drive
cantilever 3-i connected to fixed portion 4-i.
Not-shown upper electrodes and lower electrodes are connected to
piezoelectric elements 5i-a, b, c, d, e, f (i=1 to 4) for applying
a drive voltage. The dielectric polarization directions of
piezoelectric elements 5i-a, b, c, d, e, f are the same.
Power supply unit 62 applies AC voltage of Equation (1) to
piezoelectric elements 5i-a, 5i-c, 5i-e on drive cantilever 3-i
(i=1 to 4) and applies AC voltage of Equation (2) to piezoelectric
elements 5i-b, 5i-d, 5i-f on drive cantilever 3-i.
As described above, in the present embodiment, drive cantilever 3-i
is bent twice, whereby the drive force by drive cantilever 3-i is
increased and consequently, the deflection angle .theta. of mirror
part 1 is increased.
In the foregoing embodiment, drive cantilever 3-i (i=1 to 4) has a
shape bent twice at 180.degree. but may have a shape bent three or
more times at 180.degree.. When the number of bends in drive
cantilever 3-i is increased, the drive force by drive cantilever
3-i is increased. Consequently, the deflection angle of mirror part
1 is increased but the resonance frequency is reduced. The number
of bends in drive cantilever 3-i is therefore adjusted in
accordance with the required resonance frequency and the required
deflection angle.
Fifth Embodiment
FIG. 18 is a diagram illustrating the front surface of the main
part of an optical scanning device 400 in a fifth embodiment.
Optical scanning device 400 in the fifth embodiment differs from
optical scanning device 100 in the second embodiment in the
following points.
Referring to FIG. 18, mirror part 1 is formed using the crystal
plane (111) of an SOI substrate which is a kind of semiconductor
substrates. The mechanical property of the crystal plane (111) of
an SOI substrate is 3-fold symmetry.
Support cantilevers 2-1 to 2-3 support a silicon mirror part 1C
swingably. Support cantilevers 2-1 to 2-3 have the same shape and
size. Support cantilevers 2-1 to 2-3 are arranged in 120.degree.
rotational symmetry with respect to the center axis (the Z axis) of
mirror part 1.
Drive cantilevers 3-1 to 3-3 are arranged to surround mirror part
1. Drive cantilevers 3-1 to 3-3 have the same shape and size. Drive
cantilevers 3-1 to 3-3 are arranged in 120.degree. rotational
symmetry with respect to the center axis of mirror part 1.
One end of drive cantilever 3-i (i=1 to 3) is connected to support
cantilever 2-i, and the other end of drive cantilever 3-i is
connected to fixed portion 4-i. Drive cantilever 3-i has a shape
bent once at 180.degree.. Drive cantilever 3-i has two
circumferential portions extending in the same direction as the
circumferential direction of mirror part 1 and a bend portion.
Piezoelectric elements 5i-a, b, c, d (i=1 to 3) are secured on
drive cantilever 3-i. Piezoelectric elements 5i-a, b, c, d have the
same shape and size.
Of the portions that constitute drive cantilever 3-i (i=1 to 3), a
first portion of two circumferential portions extending in the same
direction as the circumferential direction of mirror part 1 has
piezoelectric elements 5i-a, b and a second portion has
piezoelectric elements 5i-c, d. Piezoelectric element 5i-a and
piezoelectric element 5i-b are spaced apart from each other.
Piezoelectric element 5i-c and piezoelectric element 5i-d are
spaced apart from each other. Piezoelectric element 5i-b and
piezoelectric element 5i-c are adjacent to each other with the bend
portion of drive cantilever 3-i interposed therebetween.
Piezoelectric element 5i-a is arranged at a position closest to one
end of drive cantilever 3-i connected to support cantilever 2-i.
Piezoelectric element 5i-d is arranged at a position closest to the
other end of drive cantilever 3-i connected to fixed portion
4-i.
Upper electrodes and lower electrodes are connected to
piezoelectric elements 5i-a, b, c, d (i=1 to 3) for applying a
drive voltage. The dielectric polarization directions of
piezoelectric elements 5i-a, b, c, d are the same.
A first axis which is the rotation axis in the natural frequency
mode 2 is parallel to mirror surface 1B. The direction of the first
axis is the direction of a straight light connecting the center of
mirror part 1 with a connection portion between mirror part 1 and
support cantilever 2-1. A second axis which is the rotation axis in
the natural frequency mode 3 is parallel to mirror surface 1B and
orthogonal to the first axis. In FIG. 18, the first axis is the X
axis, and the second axis is the Y axis.
Power supply unit 62 applies AC voltage of Equation (3) to
piezoelectric elements 5i-a, 5i-c on drive cantilever 3-i, where
i=1 to 3. Vi(1)=Vs.times.sin(.omega.t+120.degree..times.(i-1))
(3)
Here, .omega.=2.pi.F0. F0 is the resonance frequency in the natural
frequency mode 2 and the resonance frequency in the natural
frequency mode 3. t is time.
Power supply unit 62 applies AC voltage of Equation (4) to
piezoelectric elements 5i-b, 5i-d on drive cantilever 3-i, where
i=1 to 3.
Vi(2)=-Vi(1)=-Vs.times.sin(.omega.t+120.degree..times.(i-1))
(4)
Since the signs of Vi(1) in Equation (3) and Vi(2) in Equation (4)
are reversed, the phase of Vi(1) in Equation (3) and the phase of
Vi(2) in Equation (4) differ by 180.degree..
In the optical scanning device in the present embodiment, since the
drive cantilevers are fewer than in the second embodiment, the
number of drive voltages to be generated in power supply unit 62
can be reduced. Consequently, the configuration of power supply
unit 62 can be simplified. In addition, since the support points at
which mirror part 1 is displaced are fewer, the deflection angle
can be easily increased.
Although the number of drive cantilevers is three in the present
embodiment, the number of drive cantilevers is not limited thereto
and may be N. Although three drive cantilevers 3-1 to 3-3 are
ordered counterclockwise, embodiments are not limited thereto, and
they may be ordered clockwise. That is, N drive cantilevers are
ordered clockwise or counterclockwise.
When the number of drive cantilevers is N, power supply unit 62
applies AC voltage of Equation (3A) to piezoelectric elements 5i-a,
5i-c on drive cantilever 3-i, where i=1 to N.
Vi(1)=Vs.times.sin(.omega.t+(360.degree./N).times.(i-1)) (3A)
Here, .omega.=2.pi..times.F0. F0 is the resonance frequency in the
natural frequency mode 2 and the resonance frequency in the natural
frequency mode 3. t is time.
Power supply unit 62 applies AC voltage of Equation (4A) to
piezoelectric elements 5i-b, 5i-d on drive cantilever 3-i, where
i=1 to N.
Vi(2)=-Vi(1)=-Vs.times.sin(.omega.t+(360.degree./N).times.(i-1))
(4A)
Since the signs of Vi(1) in Equation (3A) and Vi(2) in Equation
(4A) are reversed, the phase of Vi(1) in Equation (3A) and the
phase of Vi(2) in Equation (4A) differ by 180.degree..
Therefore, in this case, the phase of AC voltage applied to the
first piezoelectric element (5i-a, b, c or d) on the i-th drive
cantilever (3-i) is larger by 360.degree./N than the phase of
voltage applied to the second piezoelectric element (5-(i-1)-a, b,
c, or d) on the (i-1)th drive cantilever (3-(i-1)).
In the present embodiment, N=3.times.1, and the shape of the front
surface (mirror surface 1B) of mirror part 1 is circular. However,
embodiments are not limited thereto. N may be 3.times.n (n is a
natural number), and the shape of the front surface of mirror part
1 (mirror surface 1B) may be circular.
(Note)
Optical scanning device 400 in the fifth embodiment has the
following features.
(8) N drive cantilevers (3-1 to 3-3) are ordered clockwise or
counterclockwise. The phase of AC voltage applied by the power
supply unit (62) to a first piezoelectric element (5-2-a, b, c, or
d) on the i-th drive cantilever (for example, 3-2) is larger by
(360.degree./N) than the phase of voltage applied to a second
piezoelectric element (5-1-a, b, c, or d) on the (i-1)th drive
cantilever (for example, 3-1). The position of the second
piezoelectric element (5-1-a, b, c, or d) on the (i-1)th drive
cantilever (3-1) is the same as the position of the first
piezoelectric element (5-2-a, b, c, or d) on the i-th drive
cantilever (3-2).
With such a configuration, AC voltage can be supplied to a
plurality of piezoelectric elements on N drive cantilevers such
that mirror part 1 can make all-round rotational displacement
smoothly.
(9) The mirror part (1) is formed using the crystal plane (111) of
a semiconductor substrate. N is 3.times.n (n is a natural number).
The shape of the mirror surface (1B) is circular.
With such a configuration, an optical scanning device having drive
cantilevers with uniform mechanical properties can be
implemented.
Sixth Embodiment
FIG. 19 is a diagram illustrating the front surface of the main
part of an optical scanning device 500 in a sixth embodiment.
Optical scanning device 500 in the sixth embodiment differs from
optical scanning device 100 in the second embodiment in the
following points.
Referring to FIG. 19, a mirror part 501 is formed using the crystal
plane (100) of an SOI substrate which is a kind of semiconductor
substrates, in the same manner as in the second embodiment.
The front surface (mirror surface 501B) and the back surface of
mirror part 501 are square.
Support cantilevers 2-1 to 2-4 support a silicon mirror part 501C
swingably. Support cantilevers 2-1 to 2-4 have the same shape and
size. Support cantilevers 2-1 to 2-4 are arranged in 90.degree.
rotational symmetry with respect to the center axis (the Z axis) of
mirror part 501.
Drive cantilevers 3-1 to 3-4 are disposed to surround mirror part
501. Drive cantilevers 3-1 to 3-4 have the same shape and size.
Drive cantilevers 3-1 to 3-4 are arranged in 90.degree. rotational
symmetry with respect to the center axis of mirror part 501.
One end of drive cantilever 3-i (i=1 to 4) is connected to support
cantilever 2-i, and the other end of drive cantilever 3-i is
connected to fixed portion 4-i. Drive cantilever 3-i has a shape
bent once at 180.degree.. Drive cantilever 3-i has two
circumferential portions extending in the same direction as the
circumferential direction of mirror part 501 and a bend
portion.
Piezoelectric elements 5i-a, b, c, d (i=1 to 4) are secured on
drive cantilever 3-i. Piezoelectric elements 5i-a, b, c, d have the
same shape and size.
Of the portions that constitute drive cantilever 3-i (i=1 to 4), a
first portion of two circumferential portions extending in the same
direction as the circumferential direction of mirror part 501 has
piezoelectric elements 5i-a, b, and a second portion has
piezoelectric elements 5i-c, d. Piezoelectric element 5i-a and
piezoelectric element 5i-b are spaced apart from each other.
Piezoelectric element 5i-c and piezoelectric element 5i-d are
spaced apart from each other. Piezoelectric element 5-i-b and
piezoelectric element 5i-c are adjacent to each other with the bend
portion of drive cantilever 3-i interposed therebetween.
Piezoelectric element 5i-a is arranged at a position closest to one
end of drive cantilever 3-i connected to support cantilever 2-i.
Piezoelectric element 5i-d is arranged at a position furthest from
the other end of drive cantilever 3-i connected to fixed portion
4-i.
Upper electrodes and lower electrodes are connected to
piezoelectric elements 5i-a, b, c, d (i=1 to 4) for applying a
drive voltage. The dielectric polarization directions of
piezoelectric elements 5i-a, b, c, d are the same.
A first axis which is the rotation axis in the natural frequency
mode 2 is parallel to mirror surface 501B. The direction of the
first axis is the direction of a straight line connecting the
center of mirror part 501 with a connection portion between mirror
part 501 and support cantilever 502-1. The second axis which is the
rotation axis in the natural frequency mode 3 is parallel to mirror
surface 501B and orthogonal to the first axis. In FIG. 19, the
first axis is the X axis, and the second axis is the Y axis.
Power supply unit 62 applies AC voltage of Equation (5) to
piezoelectric elements 5i-a, 5i-c on drive cantilever 3-i, where
i=1 to 4. Vi(1)=Vs.times.sin(.omega.t+90.degree..times.(i-1))
(5)
Here, .omega.=2.pi..times.F0. F0 is the resonance frequency in the
natural frequency mode 2 and the resonance frequency in the natural
frequency mode 3. t is time.
Power supply unit 62 applies AC voltage of Equation (6) to
piezoelectric elements 5i-b, 5i-d on drive cantilever 3-i, where
i=1 to 4.
Vi(2)=-Vi(1)=-Vs.times.sin(.omega.t+90.degree..times.(i-1)) (6)
Since the signs of Vi(1) in Equation (5) and Vi(2) in Equation (6)
are reversed, the phase of Vi(1) in Equation (5) and the phase of
Vi(2) in Equation (6) differ by 180.degree..
In the present embodiment, N=4.times.1, and the shape of the front
surface (mirror surface 501B) of mirror part 501 is a regular
(4.times.1) polygon. However, embodiments are not limited thereto.
N may be 4.times.n (n is a natural number), and the shape of the
front surface (mirror surface 501B) of mirror part 501 may be a
regular (4.times.n) polygon.
(Note)
Optical scanning device 500 in the sixth embodiment has the
following features.
(10) A mirror part (1) is formed using the crystal plane (100) of a
semiconductor substrate. N is 4.times.n (n is a natural number).
The shape of the mirror surface (501B) is a regular (4.times.n)
polygon.
With such a configuration, the mechanical properties of a plurality
of drive cantilevers can be made uniform.
Seventh Embodiment
FIG. 20 is a diagram illustrating the front surface of the main
part of an optical scanning device 600 in a seventh embodiment.
A mirror part 601 is formed using the crystal plane (111) of an SOI
substrate which is a kind of semiconductor substrates. The
mechanical property of the crystal plane (111) of an SOI substrate
is 3-fold symmetry.
The front surface (mirror surface 601B) and the back surface of
mirror part 601 are in the shape of a regular triangular.
Support cantilevers 2-1 to 2-3 support a silicon mirror part 601C
swingably. Support cantilevers 2-1 to 2-3 have the same shape and
size. Support cantilevers 2-1 to 2-3 are arranged in 120.degree.
rotational symmetry with respect to the center axis (the Z axis) of
mirror part 601.
Drive cantilevers 3-1 to 3-3 are arranged to surround mirror part
601. Drive cantilevers 3-1 to 3-3 have the same shape and size.
Drive cantilevers 3-1 to 3-3 are arranged in 120.degree. rotational
symmetry with respect to the center axis of mirror part 601.
One end of drive cantilever 3-i (i=1 to 3) is connected to support
cantilever 2-i, and the other end of drive cantilever 3-i is
connected to fixed portion 4-i. Drive cantilever 3-i has a shape
bent once at 180.degree.. Drive cantilever 3-i has two
circumferential portions extending in the same direction as the
circumferential direction of mirror part 601 and a bend
portion.
Piezoelectric elements 5i-a, b, c, d (=1 to 3) are secured on drive
cantilever 3-i. Piezoelectric elements 5i-a, b, c, d have the same
shape and size. Of the portions that constitute drive cantilever
3-i, a first portion of two circumferential portions extending in
the same direction as the circumferential direction of mirror part
601 has piezoelectric elements 5i-a, b, and a second portion has
piezoelectric elements 5i-c, d. Piezoelectric element 5i-a and
piezoelectric element 5i-b are spaced apart from each other.
Piezoelectric element 5i-c and piezoelectric element 5i-d are
spaced apart from each other. Piezoelectric element 5i-b and
piezoelectric element 5i-c are adjacent to each other with the bend
portion of drive cantilever 3-i interposed therebetween.
Piezoelectric element 5i-a is arranged at a position closest to one
end of drive cantilever 3-i connected to support cantilever 2-i.
Piezoelectric element 5i-d is arranged at a position closest to the
other end of drive cantilever 3-i connected to fixed portion
4-i.
Upper electrodes and lower electrodes are connected to
piezoelectric elements 5i-a, b, c, d (i=1 to 3) for applying a
drive voltage. The dielectric polarization directions of
piezoelectric elements 5i-a, b, c, d are the same.
A first axis which is the rotation axis in the natural frequency
mode 2 is parallel to mirror surface 601B. The direction of the
first axis is the direction of a straight line connecting the
center of mirror part 601 with a connection portion between mirror
part 601 and support cantilever 602-2. A second axis which is the
rotation axis in the natural frequency mode 3 is parallel to mirror
surface 601B and orthogonal to the first axis. In FIG. 20, the
first axis is the Y axis, and the second axis is the X axis.
Power supply unit 62 applies a sinusoidal voltage of Equation (7)
to piezoelectric elements 5i-a, 5i-c on drive cantilever 3-i, where
i=1 to 3. Vi(1)=Vs.times.sin(.omega.t+120.degree..times.(i-1))
(7)
Here, .omega.=2.pi..times.F0. F0 is the resonance frequency in the
natural frequency mode 2 and the resonance frequency in the natural
frequency mode 3. t is time.
Power supply unit 62 applies a sinusoidal voltage of Equation (8)
to piezoelectric elements 5i-b, 5i-d on drive cantilever 3-i, where
i=1 to 3.
Vi(2)=-Vi(1)=-Vs.times.sin(.omega.t+120.degree..times.(i-1))
(8)
Since the signs of Vi(1) in Equation (7) and Vi(2) in Equation (8)
are reversed, the phase of Vi(1) in Equation (7) and the phase of
Vi(2) in Equation (8) differ by 180.degree..
In the present embodiment, N=3.times.1, and the front surface
(mirror surface 601B) of mirror part 601 is a regular (3.times.1)
polygon. However, embodiments are not limited thereto. N may be
3.times.n (n is a natural number), and the shape of the front
surface (mirror surface 601B) of mirror part 601 may be a regular
(3.times.n) polygon.
In the present embodiment, three support cantilevers 2-1 to 2-3 are
arranged in (360.degree./3) rotational symmetry with respect to the
center axis of mirror part 601, three drive cantilevers 3-1 to 3-3
are arranged in (360.degree./3) rotational symmetry with respect to
the center axis of mirror part 601, and the shape of mirror part
601 is arranged in (360.degree./3) rotational symmetry with respect
to the center axis of mirror part 601. However, embodiments are not
limited thereto. N support cantilevers may be arranged in
(360.degree./N) rotational symmetry with respect to the center axis
of mirror part 601, N drive cantilevers may be arranged in
(360.degree./N) rotational symmetry with respect to the center axis
of mirror part 601, and the shape of mirror part 601 may be
(360.degree./N) rotational symmetric with respect to the center
axis of mirror part 601.
(Note)
Optical scanning device 600 in the seventh embodiment has the
following features.
(11) A mirror part (601) is formed by using the crystal plane (111)
of a semiconductor substrate. N is 3.times.n (n is a natural
number). The shape of the mirror surface (601B) is a regular
(3.times.n) polygon.
With such a configuration, an optical scanning device having drive
cantilevers with uniform mechanical properties can be
implemented.
(12) N support cantilevers (2-1 to 2-3) are arranged in
(360.degree./N) rotational symmetry with respect to the center axis
of mirror part (601). N drive cantilevers (3-1 to 3-3) are arranged
in (360.degree./N) rotational symmetry with respect to the center
axis of mirror part (601). The shape of the mirror part (601) is in
(360.degree./N) rotational symmetry with respect to the center axis
of mirror part (601).
With such a configuration, the resonance frequency in the first
natural frequency mode (natural frequency mode 2) can be matched
with the resonance frequency in the second natural frequency mode
(natural frequency mode 3). As a result, the mirror part (1) can
make all-round rotational displacement smoothly.
Eighth Embodiment
In a MEMS mirror capable of all-round rotation, rotational
displacement with high linearity is required because if linearity
is poor due to distorted rotational displacement, an error occurs
in the acquired distance information. The devices described in PTLs
1 to 3 do not have a mechanism controlling the linearity of mirror
displacement. The optical scanning device in the present embodiment
controls the linearity of mirror displacement.
First of all, the linearity of all-round rotational displacement is
described.
In the circumference of mirror part 1, let the inscribed angle
.PSI. at a point of largest displacement in the Z axis direction be
the mirror largest displacement inscribed angle .PSI.m. Let the
phase of AC voltage applied to piezoelectric elements 5i-a, b, c, d
be the applied voltage phase .PHI..
FIG. 21 is a diagram illustrating change in the mirror largest
displacement inscribed angle .PSI.m against change in the applied
voltage phase .PHI. in displacement of three patterns.
In FIG. 21, the horizontal axis shows the applied voltage phase
.PHI. (0.degree. to 360.degree. (one cycle)). The vertical axis
shows the mirror largest displacement inscribed angle .PSI.m. When
the angular velocity of applied voltage is .omega., .PHI.=.omega.t
(t: time).
The plot line A is a line connecting plotted points of sample data
measured in the case of all-round rotational displacement as in the
second embodiment. The plot line A is a downward straight line.
That is, the mirror largest displacement inscribed angle .PSI.m
decreases linearly with increase in the applied voltage phase
.PHI.. The high linearity shows that mirror part 1 is rotationally
displaced smoothly. This is because the position of largest
displacement on the circumference of mirror part 1 changes at
regular intervals with time. The downward slope shows rotational
displacement counterclockwise.
The plot line B is a line connecting the plotted points of sample
data measured in the case of serpentine rotational displacement.
The plot line B deviates from a straight line.
The plot line C is a line connecting the plotted points of sample
data measured in the case of vibration displacement around a single
axis, rather than rotational displacement. The plot line C is not a
downward slope. The plot line C shows displacement such that the
mirror largest displacement inscribed angle .PSI.m changes in a
range of 180.degree. to 360.degree..
FIG. 22 is a diagram illustrating the plot line B in FIG. 21 and an
approximate straight line of the plot line B.
A rotation linearity error is used as the quantity that represents
a deviation of the plot line B from the linearity of all-round
rotational displacement of the mirror part (1). The approximate
straight line is represented by Equation (9). The approximate
straight line can be obtained by the least square method.
.psi.b(.PHI.)=a.times..PHI.+b (9)
.psi.m(.PHI.) is the actually measured value. The amount of
deviation .DELTA.(.PHI.) is represented by Equation (10).
.DELTA.(.PHI.)=abs((.psi.m(.PHI.)-.psi.b(.PHI.))/(360.times.a))
(10)
Here, abs(s) is the absolute value of s.
That is, the amount of deviation .DELTA.(.PHI.) is obtained by
standardizing the difference between the actually measured value
and the approximate straight line by one-cycle full scale
(360.times.a) of the mirror largest inscribed angle .psi.m. FIG. 23
is a diagram illustrating the rotation linearity error of the plot
line B.
The maximum value of the amount of deviation .DELTA.(.PHI.) is
defined as a rotation linearity error. FIG. 23 shows a value
obtained by converting the amount of deviation .DELTA.(.PHI.)
having a value equal to or greater than zero and equal to or
smaller than one into percentage. In the example in FIG. 23, the
rotation linearity error is 8% (0.08). As the rotation linearity
error approaches zero, the linearity is higher and the all-round
rotational displacement is smoother. For example, the mean value or
the mean square value of the amount of deviation .DELTA.(.PHI.) may
be used as a rotation linearity error, rather than using the
maximum value of the amount of deviation .DELTA.(.PHI.) as a
rotation linearity error.
As shown in the second embodiment, in order to obtain a large
displacement with a small voltage, the optical scanning device in
the present embodiment applies AC voltage at the matched resonance
frequency F0 in two natural frequency modes to piezoelectric
elements 5i-a to d (i=1 to 4) to allow mirror part 1 to make
all-round rotational displacement. However, when a difference in
resonance frequency in two natural frequency modes occurs due to
manufacturing error or the like, the displacement is such as the
plot line B or the plot line C. Consequently, smooth all-round
rotational displacement with good linearity is unable to be
achieved.
In the eighth embodiment, in order to solve this problem, a
plurality of drive power supplies are used, which can independently
adjust the amplitude and the initial phase of output voltage.
FIG. 24 is a diagram for explaining voltage for driving
piezoelectric elements 5-i-a to d (i=1 to 4) in an optical scanning
device 700 in the eighth embodiment. Power supply unit 62 includes
a drive power supply 20-i for driving piezoelectric elements 5i-a,
b, c, d on drive cantilever 3-i (i=1 to 4).
Drive power supply 20-i (i=1 to 4) applies AC voltage to 5i-a, c,
of four piezoelectric elements 5i-a to d on drive cantilever 3-i,
and applies AC voltage with the reversed sign to 5i-b, d. The phase
and the amplitude of AC voltage of drive power supply 20-i can be
controlled independently of the other drive power supplies. The
frequencies of AC voltages of drive power supplies 20-1 to 20-4 are
the same but can be set to a value in accordance with the resonance
frequency.
The displacement of mirror part 1 when the resonance frequency in
the natural frequency mode 2 and the resonance frequency in the
natural frequency mode 3 do not match will now be described. It is
assumed that AC voltages shown in Equation (1) and Equation (2) are
applied in a state in which the resonance frequency in the natural
frequency mode 2 and the resonance frequency in the natural
frequency mode 3 do not match, for example, for the reason that
mirror part 1 is out of symmetry.
FIG. 25 is a diagram illustrating change in the mirror largest
displacement inscribed angle .PSI.m against change in the applied
voltage phase .PHI. when AC voltage at the resonance frequency F2
in the natural frequency mode 2 is applied. FIG. 26 is a diagram
illustrating the relation between the applied voltage phase .PHI.
and the mirror largest displacement inscribed angle .PSI.m when AC
voltage at the resonance frequency F3 in the natural frequency mode
3 is applied. FIG. 27 is a diagram illustrating the relation
between the applied voltage phase .PHI. and the mirror largest
displacement inscribed angle .PSI.m when AC voltage at the
intermediate frequency between the resonance frequency F2 in the
natural frequency mode 2 and the resonance frequency F3 in the
natural frequency mode 3 is applied.
As shown in FIG. 25 and FIG. 26, in the resonance frequency F2 in
the natural frequency mode 2 and the resonance frequency F3 in the
natural frequency mode 3, the line (plot line) connecting the plots
of the applied voltage phase .PHI. and the mirror largest
displacement inscribed angle .PSI.m has the down-sloping
characteristics. This plot line is not a straight line. This
indicates serpentine rotational displacement.
As shown in FIG. 27, in the intermediate frequency (F2+F3)/2
(hereinafter referred to as intermediate frequency Fm between the
natural frequency mode 2 and the natural frequency mode 3) between
the resonance frequency F2 in the natural frequency mode 2 and the
resonance frequency F3 in the natural frequency mode 3, the line
(plot line) connecting the plots of the applied voltage phase .PHI.
and the mirror largest displacement inscribed angle .PSI.m has the
down-sloping characteristics. This plot line indicates rotational
displacement in a reverse direction.
FIG. 28 is a diagram illustrating the relation between the
frequency of drive voltage and the rotation linearity error.
As shown in FIG. 28, in the intermediate frequency Fm between the
natural frequency mode 2 and the natural frequency mode 3, the
rotation linearity error is extremely large.
In the present embodiment, the rotation linearity error is reduced
by setting the frequency of applied AC voltage to the intermediate
frequency Fm between the natural frequency mode 2 and the natural
frequency mode 3 and adjusting the initial phase and the amplitude
of the applied AC voltage. The rotation linearity error can be
reduced also by setting the frequency of applied AC voltage to the
resonance frequency F2 in the natural frequency mode 2 or the
resonance frequency F3 in the natural frequency mode 3 and
adjusting the initial phase and the amplitude of the applied AC
voltage, but it is impossible to increase the deflection angle
.theta. of mirror part 1.
When the rotation linearity error is reduced by adjusting the
initial phase and the amplitude of AC voltage using the resonance
frequency F2 in the natural frequency mode 2, the deflection angle
.theta. is determined by displacement in the natural frequency mode
3. Since the displacement in the Z axis direction in the natural
frequency mode 3 is small, the deflection angle .theta. is small.
When the rotation linearity error is reduced by adjusting the
initial phase and the amplitude of AC voltage using the resonance
frequency F3 in the natural frequency mode 3, the deflection angle
.theta. is determined by displacement in the natural frequency mode
2. Since the displacement in the Z axis direction in the natural
frequency mode 2 is small, the deflection angle .theta. is small.
Accordingly, in the present embodiment, in order to reduce the
rotation linearity error and maximize the deflection angle .theta.,
the intermediate frequency Fm between the resonance frequency in
the natural frequency mode 2 and the resonance frequency in the
natural frequency mode 3 is used.
Drive power supply 20-i (i=1 to 4) outputs AC voltages Vi(1), Vi(2)
as follows. V1(1)=Vs1.times.sin(.omega.t+.PHI.1) (11)
V1(2)=-Vs1.times.sin(.omega.t+.PHI.1) (12)
V2(1)=Vs2.times.sin(.omega.t+90.degree.+.PHI.2) (13)
V2(2)=-Vs2.times.sin(.omega.t+90.degree.+.PHI.2) (14)
V3(1)=Vs3.times.sin(.omega.t+180.degree.+.PHI.3) (15)
V3(2)=-Vs3.times.sin(.omega.t+180.degree.+.PHI.3) (16)
V4(1)=Vs4.times.sin(.omega.t+270.degree.+.PHI.4) (17)
V4(2)=-Vs4.times.sin(.omega.t+270.degree.+.PHI.4) (18)
Here, .omega.=2.pi..times.(F2+F3)/2. F2 is the resonance frequency
in the natural frequency mode 2, and F3 is the resonance frequency
in the natural frequency mode 3. t is time.
FIG. 29 is a flowchart illustrating the procedure for adjusting
voltages of drive power supplies 20-1 to 20-4 for allowing mirror
part 1 to make all-round rotational displacement at the
intermediate frequency Fm between the natural frequency mode 2 and
the natural frequency mode 3 in optical scanning device 700 in the
eighth embodiment.
In step S101, control unit 61 fixes the amplitudes of voltages Vs1
to Vs4 of drive power supplies 20-1 to 20-4 to a constant value
F0.
In step S102, control unit 61 fixes the initial phases .PHI.2,
.PHI.3, .PHI.4 of voltages of drive power supplies 20-2 to 20-4 to
a constant value (=0) and measures the rotation linearity error
while changing the voltage initial phase .PHI.1 of drive power
supply 20-1. FIG. 30(a) is a diagram illustrating the relation
between the initial phase .PHI.1 and the measured rotation
linearity error in step S102.
In step S103, control unit 61 sets .PHI.1 at which the rotation
linearity error is smallest as .PHI.1d.
In step S104, control unit 61 fixes the initial phase .PHI.1 of
voltage of drive power supply 20-1 to .PHI.1d, fixes the initial
phases .PHI.3, .PHI.4 of voltages of drive power supplies 20-3,
20-4 to a constant value (=0), and measures the rotation linearity
error while changing the initial phase .PHI.2 of voltage of drive
power supply 20-2. FIG. 30(b) is a diagram illustrating the
relation between the initial phase .PHI.2 and the measured rotation
linearity error in step S104.
In step S105, control unit 61 sets .PHI.1 at which the rotation
linearity error is smallest as .PHI.2d.
In step S106, control unit 61 fixes the initial phases .PHI.1,
.PHI.2 of voltages of drive power supplies 20-1, 20-2 as .PHI.1d,
.PHI.2d, fixes the initial phase .PHI.4 of voltage of drive power
supply 20-4 to a constant value (=0), and measures the rotation
linearity error while changing the initial phase .PHI.3 of voltage
of drive power supply 20-3. FIG. 30(c) is a diagram illustrating
the relation between the initial phase .PHI.3 and the measured
rotation linearity error in step S106.
In step S107, control unit 61 sets 13 at which the rotation
linearity error is smallest as .PHI.3d.
In step S108, control unit 61 fixes the initial phases .PHI.1,
.PHI.2, .PHI.3 of voltages of drive power supplies 20-1, 20-2, 20-3
to .PHI.1d, .PHI.2d, .PHI.3d and measures the rotation linearity
error while changing the initial phase .PHI.4 of voltage of drive
power supply 20-4. FIG. 30(d) is a diagram illustrating the
relation between the initial phase .PHI.4 and the measured rotation
linearity error in step S108.
In step S109, control unit 61 sets .PHI.4 at which the rotation
linearity error is smallest as .PHI.4d.
In step S110, control unit 61 fixes the initial phases .PHI.1,
.PHI.2, .PHI.3, .PHI.4 of voltages of drive power supplies 20-1 to
20-4 to .PHI.1d, .PHI.2d, .PHI.3d, .PHI.4d.
In step S111, control unit 61 fixes the amplitudes Vs2, Vs3, Vs4 of
voltages of drive power supplies 20-2, 20-3, 20-4 to a constant
value V0 and measures the rotation linearity error while changing
the amplitude Vs1 of voltage of drive power supply 20-1. FIG. 30(e)
is a diagram illustrating the relation between the amplitude Vs1
and the measured rotation linearity error in step S111.
In step S112, control unit 61 sets Vs1 at which the rotation
linearity error is smallest as Vs1d.
In step S113, control unit 61 fixes the amplitudes Vs3, Vs4 of
voltages of drive power supplies 20-3, 20-4 to a constant value V0,
fixes the amplitude Vs1 of voltage of drive power supply 20-1 to
Vs1d, and measures the rotation linearity error while changing the
amplitude Vs2 of voltage of drive power supply 20-2. FIG. 30(f) is
a diagram illustrating the relation between the amplitude Vs2 and
the measured rotation linearity error in step S113.
In step S114, control unit 61 sets Vs2 at which the rotation
linearity error is smallest as Vs2d.
In step S115, control unit 61 fixes the amplitude Vs4 of voltage of
drive power supply 20-4 to a constant value V0, fixes the
amplitudes Vs1, Vs2 of voltages of drive power supplies 20-1, 20-2
to Vs1d, Vs2d, and measures the rotation linearity error while
changing the amplitude Vs3 of voltage of drive power supply 20-3.
FIG. 30(g) is a diagram illustrating the relation between the
amplitude Vs3 and the measured rotation linearity error in step
S115.
In step S116, control unit 61 sets Vs3 at which the rotation
linearity error is smallest as Vs3d.
In step S117, control unit 61 fixes the amplitudes Vs1, Vs2, Vs3 of
voltages of drive power supplies 20-1, 20-2, 20-3 to Vs1d, Vs2d,
Vs3d and measures the rotation linearity error while changing the
amplitude Vs4 of voltage of drive power supply 20-4. FIG. 30(h) is
a diagram illustrating the relation between the amplitude Vs4 and
the measured rotation linearity error in step S117.
In step S118, control unit 61 sets Vs4 at which the rotation
linearity error is smallest as Vs4d.
After the adjustment described above, control unit 61 sets the
initial phases .PHI.1 to .PHI.4 in Equations (11) to (18) to
.PHI.1d to .PHI.4d and sets the amplitudes Vs1 to Vs4 to Vs1d to
Vs4d to drive mirror part 1.
FIG. 31 is a diagram illustrating the applied voltage phase .PHI.
and the measured mirror largest displacement inscribed angle .PSI.m
after the adjustment of amplitude and initial phase of output
voltage of drive power supply 20-i (i=1 to 4).
It can be understood that by controlling the initial phases and the
amplitudes of drive power supplies 20-1 to 20-4, mirror part 1,
which has rotated with a distortion in the opposite direction
before the adjustment, rotates smoothly with good linearity.
Control unit 61 can also control the deflection angle .theta. of
mirror part 1 by fixing the initial phases of voltages of drive
power supplies 20-1, 20-2, 20-3, 20-4 to .PHI.1d, .PHI.2d, .PHI.3d,
.PHI.4d and setting the amplitudes to k.times.Vs1d, k.times.Vs2d,
k.times.Vs3d, k.times.Vs4d, and changing k.
(Note)
Optical scanning device 700 in the eighth embodiment has the
following features.
(13) An optical scanning device (700) includes a mirror part (1)
having a mirror surface (1B) configured to reflect light, N
(N.gtoreq.3) support cantilevers (2-1 to 2-4) supporting the mirror
part (1) swingably, and N drive cantilevers (3-1 to 3-4)
respectively connected to N support cantilevers (2-1 to 2-4). N
drive cantilevers (3-1 to 3-4) are arranged to surround mirror part
1. An end of both ends of each of N drive cantilevers (3-1 to 3-4)
that is not connected to the support cantilever (2-1 to 2-4) is
fixed. Each of N drive cantilevers (3-1 to 3-4) has a shape bent
one or more times at 180.degree.. The optical scanning device (700)
further includes a plurality of driving piezoelectric elements
(5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a to d) secured on N drive
cantilevers (3-1 to 3-4) and a power supply unit (62) configured to
apply AC voltage to a plurality of piezoelectric elements (5-1-a to
d, 5-2-a to d, 5-3-a to d, 5-4-a to d). N support cantilevers (2-1
to 2-4) are arranged in (360.degree./N) rotational symmetry with
respect to the center axis of the mirror part (1). The mirror part
(1) has a first natural frequency mode (natural frequency mode 2)
of being rotationally displaced around a first axis (the X axis)
and a second natural frequency mode (natural frequency mode 3) of
being rotationally displaced around a second axis (the Y axis). The
first axis (the X axis) and the second axis (the Y axis) are
parallel to the mirror surface (1B). The direction of the first
axis (the X axis) is the direction of a straight line connecting
the center of the mirror part (1) with a connection portion between
the mirror part (1) and one of N support cantilevers (2-1 to 2-4).
The second axis (the Y axis) is orthogonal to the first axis (the X
axis). The power supply unit (62) applies AC voltage at an
intermediate frequency (Fm) between a resonance frequency F2 in the
first natural frequency mode (natural frequency mode 2) and a
resonance frequency F3 in the second natural frequency mode
(natural frequency mode 3). The amplitude and the initial phase of
AC voltage can be adjusted.
Accordingly, even when the resonance frequency F2 in the first
natural frequency mode (natural frequency mode 2) and the resonance
frequency F3 in the second natural frequency mode (natural
frequency mode 3) differ, the mirror part (1) can make all-round
rotational displacement smoothly because the frequency of AC
voltage to a plurality of piezoelectric elements is (F2+F3)/2, and
the amplitude and the initial phase of AC voltage can be
adjusted.
(14) The power supply unit (62) includes N drive power supplies
(20-i) (i=1 to 4) each configured to apply AC voltage to a
plurality of piezoelectric elements (5i-a to d) on the
corresponding drive cantilever (3-i).
The amplitude and the initial phase of AC voltage applied to a
plurality of piezoelectric elements therefore can be adjusted by
individually adjusting the output voltages of N drive power
supplies.
(15) In adjustment of the initial phase of AC voltage, the control
unit (61) selects one drive cantilever (for example, 3-2) from
among N drive cantilevers (3-1 to 3-4), fixes the amplitude and the
initial phase of AC voltage applied to a plurality of piezoelectric
elements (5-1-a to d, 5-3-a to d, 5-4-a to d) on one or more drive
cantilevers (3-1, 3-3, 3-4) other than the selected drive
cantilever, fixes the amplitude (Vs2) of AC voltage applied to a
plurality of piezoelectric elements (5-2-a to d) on the selected
drive cantilever (3-2), and changes the initial phase (.PHI.2) and
determines the initial phase (.PHI.2) when the rotation linearity
error is smallest as an adjustment value (.PHI.2d) of the initial
phase of AC voltage applied to a plurality of piezoelectric
elements (5-2-a to d) on the selected drive cantilever (3-2). The
rotation linearity error is the quantity that represents a
deviation from the linearity of all-round rotational displacement
of the mirror part (1). The all-round rotational displacement of
mirror part 1 is a displacement of the mirror part (1) such that
the center axis of the mirror part (1) makes a turn while the
deflection angle (.theta.) of the mirror part (1) is kept
constant.
Therefore, even when the resonance frequency F2 in the first
natural frequency mode (natural frequency mode 2) and the resonance
frequency F3 in the second natural frequency mode (natural
frequency mode 3) differ, the mirror part (1) can make all-round
rotational displacement smoothly by setting the frequency of AC
voltage to a plurality of piezoelectric elements to (F2+F3)/2 and
adjusting the initial phase such that rotation linear error is
reduced.
(16) In adjustment of the amplitude of AC voltage, the control unit
(61) selects one drive cantilever (for example, 3-2) from among N
drive cantilevers (3-1 to 3-4), fixes the amplitude and the initial
phase of AC voltage applied to a plurality of piezoelectric
elements (5-1-a to d, 5-3-a to d, 5-4-a to d) on one or more drive
cantilevers (3-1, 3-3, 3-4) other than the selected drive
cantilever, fixes the initial phase (.PHI.2) of AC voltage applied
to a plurality of piezoelectric elements (5-2-a to d) on the
selected drive cantilever (3-2), and changes the amplitude (Vs2)
and determines the amplitude (Vs2) at which the rotation linearity
error is smallest, as an adjustment value (Vs2d) of amplitude of AC
voltage applied to a plurality of piezoelectric elements (5-2-a to
d) on the selected drive cantilever (3-2).
Therefore, even when the resonance frequency F2 in the first
natural frequency mode (natural frequency mode 2) and the resonance
frequency F3 in the second natural frequency mode (natural
frequency mode 3) differ, the mirror part (1) can make all-round
rotational displacement smoothly by setting the frequency of AC
voltage to a plurality of piezoelectric elements to (F2+F3)/2 and
adjusting the amplitude such that the rotation linearity error is
reduced.
Ninth Embodiment
In a ninth embodiment, the rotation linearity error is reduced by
setting the frequency of AC voltage applied to piezoelectric
elements to an intermediate frequency Fm between the natural
frequency mode 2 and the natural frequency mode 3 and adjusting the
initial phase and the amplitude of the applied AC voltage in the
same manner as in the eighth embodiment.
In the ninth embodiment, control unit 61 fixes the initial phase
.PHI.4 of drive power supply 20-4 to a constant value (=0) and
fixes the amplitude Vs4 to a constant value V0. The reason why the
initial phase and the amplitude of output voltage of drive power
supply 20-4 are not adjusted after the initial phases and the
amplitudes of output voltages of drive power supplies 20-1 to 20-3
are adjusted and fixed is that the adjustment may increase the
rotation linearity error.
FIG. 32 is a flowchart illustrating the procedure for adjusting
output voltages of drive power supplies 20-1 to 20-4 for allowing
mirror part 1 to make all-round rotational displacement at the
intermediate frequency Fm between the natural frequency mode 2 and
the natural frequency mode 3 in an optical scanning device 800 in
the ninth embodiment.
This flowchart differs from the flowchart in FIG. 29 in that it
does not include steps S108, S117 but includes step S209 instead of
step S109 and includes step S218 instead of step S118.
In step S209, control unit 61 sets .PHI.4d to a constant value
(=0).
In step S218, control unit 61 sets Vs4d to a constant value V0.
As described above, the present embodiment can prevent increase in
rotation linearity error by not adjusting the initial phase and the
amplitude of output voltage of drive power supply 20-4.
Tenth Embodiment
FIG. 33 is a diagram for explaining voltage to drive piezoelectric
elements 5i-a to d (i=1 to 4) in an optical scanning device 900 in
a tenth embodiment. Power supply unit 62 includes a drive power
supply 20-1 and a drive power supply 20-2.
Drive power supply 20-1 supplies AC voltage to piezoelectric
elements 5-1-a to d, 5-3-a to d on drive cantilevers 3-1, 3-3.
Drive power supply 20-2 supplies AC voltage to piezoelectric
elements 5-2-a to d, 5-4-a to d on drive cantilevers 3-2, 3-4.
The phase and the amplitude of AC voltage of drive power supply
20-i can be controlled independently of the other drive power
supply. The frequencies of AC voltages of drive power supplies
20-1, 20-2 are the same but can be set to a value in accordance
with the resonance frequency.
Also in the present embodiment, the phase and the amplitude of AC
voltage of drive power supply 20-i are adjusted when the resonance
frequency F2 in the natural frequency mode 2 and the resonance
frequency F3 in the natural frequency mode 3 do not match, in the
same manner as in the eighth and ninth embodiments.
Drive power supply 20-1 outputs AC voltages Vi(1), Vi(2) as
follows. V1(1)=Vs1.times.sin(.omega.t+.PHI.1) (19)
V1(2)=-Vs1.times.sin(.omega.t+.PHI.1) (20)
Here, .omega.=2.pi..times.(F2+F3)/2. F2 is the resonance frequency
in the natural frequency mode 2, and F3 is the resonance frequency
in the natural frequency mode 3. t is time.
Drive power supply 20-2 outputs AC voltages V2(1), V2(2) as
follows. V2(1)=Vs2.times.sin(.omega.t+90.degree.+.PHI.2) (21)
V2(2)=-Vs2.times.sin(.omega.t+90.degree.+.PHI.2) (22)
V1(1) is supplied to piezoelectric elements 5-1-a, 5-1-c on the
first drive cantilever 3-1 and supplied to piezoelectric elements
5-3-b, 5-3-d on the third drive cantilever 3-3.
V1(2) is supplied to piezoelectric elements 5-1-b, 5-1-d on the
first drive cantilever 3-1 and supplied to piezoelectric elements
5-3-a, 5-3-c on the third drive cantilever 3-3.
V2(1) is supplied to piezoelectric elements 5-2-a, 5-2-c on the
second drive cantilever 3-2 and supplied to piezoelectric elements
5-4-b, 5-4-d on the fourth drive cantilever 3-4.
V2(2) is supplied to piezoelectric elements 5-2-b, 5-2-d on the
second drive cantilever 3-2 and supplied to piezoelectric elements
5-4-a, 5-4-c on the fourth drive cantilever 3-4.
FIG. 34 is a flowchart illustrating the procedure for adjusting
voltages of drive power supplies 20-1, 20-2 for allowing mirror
part 1 to make all-round rotational displacement at the
intermediate frequency Fm between the natural frequency mode 2 and
the natural frequency mode 3 in optical scanning device 900 in the
tenth embodiment.
In step S301, control unit 61 fixes the amplitudes Vs1, Vs2 of
voltages of drive power supplies 20-1, 20-2 to a constant value
V0.
In step S302, control unit 61 fixes the initial phase .PHI.2 of
voltage of drive power supply 20-2 to a constant value (=0) and
measures the rotation linearity error while changing the initial
phase .PHI.1 of voltage of drive power supply 20-1. FIG. 35(a) is a
diagram illustrating the relation between the initial phase .PHI.1
and the measured rotation linearity error in step S302.
In step S303, control unit 61 sets .PHI.1 at which the rotation
linearity error is smallest as .PHI.1d.
In step S304, control unit 61 sets .PHI.2d to a constant value
(=0).
In step S305, control unit 61 fixes the initial phases .PHI.1,
.PHI.2 of voltages of drive power supplies 20-1, 20-2 to .PHI.1d,
.PHI.2d.
In step S306, control unit 61 fixes the amplitude Vs2 of voltage of
drive power supply 20-2 to a constant value V0 and measures the
rotation linearity error while changing the amplitude Vs1 of
voltage of drive power supply 20-1. FIG. 35(b) is a diagram
illustrating the relation between the amplitude Vs1 and the
measured rotation linearity error in step S306.
In step S307, control unit 61 sets Vs1 at which the rotation
linearity error is smallest as Vs1d.
In step S218, control unit 61 sets Vs4d to constant value V0.
After the adjustment described above, control unit 61 sets the
initial phases .PHI.1, .PHI.2 in Equations (19) to (22) to .PHI.1d,
.PHI.2d and sets the amplitudes Vs1, Vs2 to Vs1d, Vs2d to drive
mirror part 1.
As described above, the reason why the initial phase and the
amplitude of output voltage of drive power supply 20-2 need not be
adjusted after the initial phase and the amplitude of output
voltage of drive power supply 20-1 are adjusted and fixed is that
the rotation linearity can be adjusted by adjusting the relative
amplitude and initial phase of the natural frequency mode 2 and the
natural frequency mode 3. The natural frequency mode 2 is
rotational vibration around the X axis, and the natural frequency
mode 3 is rotational vibration around the Y axis. The vibration
characteristics of the natural frequency mode 2 can be adjusted by
adjusting the output voltage of drive power supply 20-1. The
vibration characteristics of the natural frequency mode 3 can be
adjusted by adjusting the output voltage of drive power supply
20-2. The amplitude and the initial phase of output voltage of
drive power supply 20-2 are initially fixed and the rotation
vibration characteristics around the Y axis are fixed, so that the
relative drive characteristics around the Y axis and around the X
axis can be adjusted by adjusting the amplitude and the phase of
output voltage of drive power supply 20-1.
Control unit 61 can control the deflection angle .theta. of mirror
part 1 by fixing the initial phases of voltages of drive power
supplies 20-1, 20-2 to .PHI.1d, .PHI.2d, setting the amplitudes to
k.times.Vs1d, k.times.Vs2d, and changing k.
The optical scanning device illustrated in the present embodiment
is advantageous in that the drive power supplies can be simplified
and the adjustment of rotational displacement can also be
simplified.
In the present embodiment, four drive cantilevers 3-1 to 3-4 are
ordered counterclockwise. However, embodiments are not limited
thereto, and they may be ordered clockwise.
(Note)
Optical scanning device 900 in the tenth embodiment has the
following features.
(17) Where N=4, four drive cantilevers are ordered clockwise or
counterclockwise. Four support cantilevers (2-1 to 2-4) are
arranged in 90.degree. rotational symmetry with respect to the
center axis of a mirror part (1). Four drive cantilevers (3-1 to
3-4) are arranged in 90.degree. rotational symmetry with respect to
the center axis of the mirror part (1). Of the portions that
constitute each (3-i) of four drive cantilevers (3-1 to 3-4), each
of a plurality of circumferential portions extending in the same
direction as the circumferential direction of the mirror part (1)
has two piezoelectric elements (5i-a and 5i-b, 5i-c and 5i-d). The
power supply unit (61) includes a first drive power supply (20-1)
configured to output a first AC voltage (V1(1)) having a first
initial phase (.PHI.1) and a first amplitude (Vs1), and a second AC
voltage (V1(2)) having a phase different from the first AC voltage
(V1(1)) by 180 degrees and a second drive power supply (20-2)
configured to output a third AC voltage (V2(1)) having a second
initial phase (.PHI.2) and a second amplitude (Vs2) and a fourth AC
voltage (V2(2)) having a phase different from the third AC voltage
(V2(1)) by 180 degrees. The first drive power supply (20-1) applies
the first AC voltage (V1(1)) to a first piezoelectric element
(5-1-a) near the support cantilever (2-1), among a plurality of
piezoelectric elements arranged on a first drive cantilever (3-1),
and applies the first AC voltage (V1(1)) or the second AC voltage
(V1(2)) to the other piezoelectric elements (5-1-b to d) arranged
on the first drive cantilever (3-1). The first drive power supply
(20-1) applies the second AC voltage (V1(2)) to a second
piezoelectric element (5-3-a) near the support cantilever (2-3),
among a plurality of piezoelectric elements arranged on a third
drive cantilever (3-3), and applies the first AC voltage (V1(1)) or
the second AC voltage (V1(2)) to the other piezoelectric elements
(5-3-b to d) arranged on the third drive cantilever (3-3). The
second drive power supply (20-2) applies the third AC voltage
(V2(1)) to a third piezoelectric element (5-2-a) near the support
cantilever (2-2), among a plurality of piezoelectric elements
arranged on a second drive cantilever (3-2), and applies the third
AC voltage (V2(1)) or the fourth AC voltage (V2(2)) to the other
piezoelectric elements (5-2-b to d) arranged on the second drive
cantilever (3-2). The second drive power supply (20-2) applies the
fourth AC voltage (V2(2)) to a fourth piezoelectric element (5-4-a)
near the support cantilever (2-4), among a plurality of
piezoelectric elements arranged on a fourth drive cantilever (3-4),
and applies the third AC voltage (V2(1)) or the fourth AC voltage
(V2(2)) to the other piezoelectric elements (5-4-b to d) arranged
on the fourth drive cantilever (3-4).
As described above, with only two drive power supplies (20-1,
20-2), AC voltage can be supplied to the piezoelectric elements
(5i-a to d: i=1 to 4) on four drive cantilevers (3-1 to 3-4).
(18) The control unit (61) fixes the second initial phase (.PHI.2)
and the second amplitude (Vs2) and fixes the first amplitude (Vs1),
and changes the first initial phase (.PHI.1) to determine the first
initial phase (.PHI.1) at which the rotation linearity error is
smallest as an adjustment value (.PHI.1d) of the first initial
phase. The control unit (61) fixes the second initial phase
(.PHI.2) and the second amplitude (Vs2) and fixes the first initial
phase (.PHI.1), and changes the first amplitude (Vs1) to determine
the first amplitude (Vs1) at which the rotation linearity error is
smallest as an adjustment value (Vs1d) of the first amplitude. The
rotation linearity error is the quantity that represents a
deviation from the linearity of all-round rotational displacement
of the mirror part (1). The all-round rotational displacement of
the mirror part (1) is a displacement of the mirror part (1) such
that the center axis of the mirror part (1) makes a turn while the
deflection angle of the mirror part (1) is kept constant.
With such a configuration, the rotation linearity error can be
reduced only by adjusting the first initial phase (.PHI.1) and the
first amplitude (Vs1) of AC voltage of the first drive power supply
(20-2).
Eleventh Embodiment
FIG. 36 is a diagram illustrating mirror part 1 of an optical
scanning device 1000 in an eleventh embodiment.
As shown in FIG. 36, mirror part 1 includes a trimming pattern 18
on the outer periphery of a silicon mirror part 1C. Trimming
pattern 18 is formed of a plurality of projections.
FIG. 37 is an enlarged view of the trimming pattern of mirror part
1.
When the difference between the resonance frequency F2 in the
natural frequency mode 2 and the resonance frequency F3 in the
natural frequency mode 3 is large, trimming pattern 18 is trimmed
by local cutting, for example, by a laser, or by evaporation to
reduce the difference between the resonance frequency F2 and the
resonance frequency F3.
FIG. 38 is a diagram illustrating the relation between the drive
frequency and the mirror largest displacement before trimming.
The mirror largest displacement refers to the largest displacement
in the Z axis direction at a point on the circumference of mirror
part 1. Since the difference between the resonance frequency F2 and
the resonance frequency F3 is large before trimming, the mirror
largest displacement at the intermediate frequency Fm is small.
FIG. 39 is a diagram illustrating the relation between the drive
frequency and the mirror largest displacement after trimming.
Since the difference between the resonance frequency F2 and the
resonance frequency F3 is small after trimming, the mirror largest
displacement at the intermediate frequency Fm is large.
When the mirror part 1 makes all-round rotational displacement with
the intermediate frequency (F2+F3)/2 between two resonance
frequencies F2 and F3, the initial phase and the amplitude of the
drive power supply are controlled as described in the eighth to
tenth embodiments to enable rotation displacement with good
linearity.
However, as shown in FIG. 38, before trimming, since there is no
displacement-increasing effect by resonance, it is impossible to
increase the deflection angle of mirror part 1. Although increasing
the amplitude of drive voltage increases the deflection angle, a
pressure exceeding the withstand pressure upper limit is exerted on
piezoelectric elements 5i-a to d, if the amplitude of drive voltage
is increased.
On the other hand, as shown in FIG. 39, after trimming, when mirror
part 1 makes all-round rotational displacement with the
intermediate frequency (F2+F3)/2 between two resonance frequencies
F2 and F3, a minute displacement of piezoelectric elements 5i-a to
d can be converted into a large displacement of drive cantilever
3-i, and the deflection angle of mirror part 1 can be
increased.
The trimming step is performed in a testing step at a wafer level
during fabrication of optical scanning device 1000 or in a testing
step after packaging.
FIG. 40 is a flowchart illustrating the procedure for adjusting
optical scanning device 1000 in the eleventh embodiment.
In step S501, power supply unit 62 applies voltages of Equations
(1), (2) to piezoelectric elements 5i-a, b, c, d (i=1 to 4), where
.omega.=2.pi.{(F3+F3)/2}.
In step S502, detecting piezoelectric element 6-i (i=1 to 4)
generates electric charge proportional to the deflection angle
.theta. of mirror part 1. Control unit 61 calculates the deflection
angle of mirror part 1 based on the electric charge generated in
detecting piezoelectric element 6-i and displays the calculated
deflection angle on a not-shown monitor.
In step S503, the tester trims a part or the whole of trimming
pattern 18 on the outer periphery of mirror part 1, for example, by
a laser, in accordance with the deflection angle of mirror part 1
displayed on the monitor.
In the present embodiment, the trimming pattern is formed of a
plurality of minute projections. However, embodiments are not
limited thereto, and the trimming pattern may have any other
shapes.
(Note)
Optical scanning device 1000 in the eleventh embodiment has the
following features.
(19) The mirror part (1) includes a trimming pattern (18) on the
outer periphery thereof.
With such a configuration, even when resonant drive with the
intermediate frequency Fm is impossible due to a large difference
between the resonance frequency F2 in the first natural frequency
mode (natural frequency mode 2) and the resonance frequency F3 in
the second natural frequency mode (natural frequency mode 3), the
trimming pattern provided on the outer periphery of the mirror part
(1) can be trimmed to reduce the difference between the resonance
frequency F2 and the resonance frequency F3, thereby enabling
resonant drive with the intermediate frequency Fm.
Twelfth Embodiment
In the eleventh embodiment, a trimming pattern is arranged on the
outer periphery of mirror part 1. However, embodiments are not
limited thereto.
In a twelfth embodiment, equivalent effects can be achieved by
removing a part of mirror part 1 of an optical scanning device
1100, a part of drive cantilevers 3-1 to 3-4, or a part of support
cantilevers 2-1 to 2-4, for example, by a laser.
FIG. 41 is a flowchart illustrating the procedure for adjusting
optical scanning device 1100 in a twelfth embodiment.
In step S501, power supply unit 62 applies voltages of Equations
(1), (2) to piezoelectric elements 5i-a to d (i=1 to 4), where
.omega.=2.pi.{(F3+F3)/2}.
In step S502, detecting piezoelectric element 6-i (i=1 to 4)
generates electric charge proportional to the deflection angle
.theta. of mirror part 1. Control unit 61 calculates the deflection
angle of mirror part 1 based on the electric charge generated in
detecting piezoelectric element 6-i and displays the calculated
deflection angle on a not-shown monitor.
In step S603, the tester trims a part of mirror part 1 (including a
part of the trimming pattern, if any), a part of drive cantilevers
3-1 to 3-4, or a part of support cantilevers 2-1 to 2-4, for
example, by a laser, in accordance with the deflection angle of
mirror part 1 displayed on the monitor.
The initial phase and the amplitude of AC voltage may be further
adjusted after trimming as in the foregoing embodiment.
(Note)
Optical scanning device 1100 in the twelfth embodiment has the
following features.
(20) In a method of adjusting an optical scanning device (1100),
the optical scanning device (1100) includes a mirror part (1)
having a mirror surface (1B) configured to reflect light, N
(N.gtoreq.3) support cantilevers (2-1 to 2-4) supporting the mirror
part (1) swingably, and N drive cantilevers (3-1 to 3-4)
respectively connected to N support cantilevers (2-1 to 2-4). N
drive cantilevers (3-1 to 3-4) are arranged to surround the mirror
part 1. An end of both ends of each of N drive cantilevers (3-1 to
3-4) that is not connected to the support cantilever (2-1 to 2-4)
is fixed. Each of N drive cantilevers (3-1 to 3-4) has a shape bent
one or more times at 180.degree.. The optical scanning device
(1100) further includes a plurality of driving piezoelectric
elements (5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a to d) secured
on N drive cantilevers (3-1 to 3-4) and a power supply unit (62). N
support cantilevers (2-1 to 2-4) are arranged in (360.degree./N)
rotational symmetry with respect to the center axis of the mirror
part (1). The mirror part (1) has a first natural frequency mode
(natural frequency mode 2) of being rotationally displaced around a
first axis (the X axis) and a second natural frequency mode
(natural frequency mode 3) of being rotationally displaced around a
second axis (the Y axis). The first axis (the X axis) and the
second axis (the Y axis) are parallel to the mirror surface (1B).
The direction of the first axis (the X axis) is the direction of a
straight line connecting the center of the mirror part (1) with a
connection portion between the mirror part (1) and one of N support
cantilevers (2-1 to 2-4). The second axis (the Y axis) is
orthogonal to the first axis (the X axis). This adjustment method
includes the step of applying AC voltage to a plurality of
piezoelectric elements (5i-a to d) by the power supply unit (62).
The phase of AC voltage is a value corresponding to the position of
the piezoelectric element, and the frequency of AC voltage is an
intermediate frequency (Fm) between a resonance frequency F2 in the
first natural frequency mode (natural frequency mode 2) and a
resonance frequency F3 in the second natural frequency mode
(natural frequency mode 3). This adjustment method further includes
the steps of: measuring the deflection angle of the mirror part
(1); and trimming a part of the mirror part (1), a part of the
support cantilevers (2-1 to 2-4), or a part of drive cantilevers
(3-1 to 3-4).
Therefore, even when resonant drive with the intermediate frequency
Fm is impossible due to a large difference between the resonance
frequency F2 in the first natural frequency mode (natural frequency
mode 2) and the resonance frequency F3 in the second natural
frequency mode (natural frequency mode 3), a part of the mirror
part (1), a part of the support cantilevers (2-1 to 2-4), or a part
of the drive cantilevers (3-1 to 3-4) is trimmed by a laser,
whereby the difference between the resonance frequency F2 and the
resonance frequency F3 is reduced to enable resonant drive with the
intermediate frequency Fm.
Thirteenth Embodiment
FIG. 42 is a diagram illustrating an optical scanning device 1200
as an example of a thirteenth embodiment.
In the present embodiment, as shown in FIG. 42, a frequency
adjusting film 31 is formed on the periphery of mirror part 1.
Frequency adjusting film 31 can be formed by locally growing a thin
film of tungsten or the like by a local film deposition method such
as laser CVD.
In the twelfth embodiment, a part of the mirror structure is
locally cut, for example, by a laser or evaporated for adjustment
of the resonance frequency F2 in the natural frequency mode 2 and
the resonance frequency F3 in the natural frequency mode 3, whereby
the difference between the resonance frequency F2 and the resonance
frequency F3 is reduced. In the present embodiment, frequency
adjusting film 31 on the periphery of mirror part 1 adjusts the
resonance frequency difference.
FIG. 43 is a diagram illustrating an optical scanning device 1300
as another example of the thirteenth embodiment.
As shown in FIG. 43, a frequency adjusting film 32 is formed on
support cantilever 2-1 to adjust the resonance frequency
difference.
The frequency adjusting film may be formed on a part of mirror part
1, a part of drive cantilevers 3-1 to 3-4, or a part of support
cantilevers 2-1 to 2-4, rather than being formed on the periphery
of mirror part 1 and on support cantilever 2-1.
FIG. 44 is a flowchart illustrating the procedure for adjusting
optical scanning devices 1200, 1300 in the thirteenth
embodiment.
In step S501, power supply unit 62 applies voltages of Equations
(1), (2) to piezoelectric elements 5i-a to d (i=1 to 4), where
.omega.=2.pi.{(F3+F3)/2}.
In step S502, detecting piezoelectric element 6-i (i=1 to 4)
generates electric charge proportional to the deflection angle
.theta. of mirror part 1. Control unit 61 calculates the deflection
angle of mirror part 1 based on the electric charge generated in
detecting piezoelectric element 6-i and displays the calculated
deflection angle on a not-shown monitor.
In step S603, the tester forms a thin film on a part of mirror part
1, a part of drive cantilevers 3-1 to 3-4, or a part of support
cantilevers 2-1 to 2-4 in accordance with the deflection angle of
mirror part 1 displayed on the monitor.
The frequency adjustment by a local thin film is advantageous in
that the adjustment range is wide and minute adjustment is
possible, compared with trimming, because the degree of freedom in
thin film formation is high. In trimming, the evaporated structure
may be deposited again to cause a resonance frequency shift,
whereas the local thin film is advantages in that the problem
caused by re-deposition does not occur.
(Note)
Optical scanning device 1200, 1300 in the thirteenth embodiment has
the following features.
(21) In a method of adjusting an optical scanning device (1200,
1300), the optical scanning device (1100) includes a mirror part
(1) having a mirror surface (1B) configured to reflect light, N
(N.gtoreq.3) support cantilevers (2-1 to 2-4) supporting the mirror
part (1) swingably, and N drive cantilevers (3-1 to 3-4)
respectively connected to N support cantilevers (2-1 to 2-4). N
drive cantilevers (3-1 to 3-4) are arranged to surround the mirror
part 1. An end of both ends of each of N drive cantilevers (3-1 to
3-4) that is not connected to the support cantilever (2-1 to 2-4)
is fixed. Each of N drive cantilevers (3-1 to 3-4) has a shape bent
one or more times at 180.degree.. The optical scanning device
(1100) further includes a plurality of driving piezoelectric
elements (5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a to d) secured
on N drive cantilevers (3-1 to 3-4) and a power supply unit (62). N
support cantilevers (2-1 to 2-4) are arranged in (360.degree./N)
rotational symmetry with respect to the center axis of the mirror
part (1). The mirror part (1) has a first natural frequency mode
(natural frequency mode 2) of being rotationally displaced around a
first axis (the X axis) and a second natural frequency mode
(natural frequency mode 3) of being rotationally displaced around a
second axis (the Y axis). The first axis (the X axis) and the
second axis (the Y axis) are parallel to the mirror surface (1B).
The direction of the first axis (the X axis) is the direction of a
straight line connecting the center of the mirror part (1) with a
connection portion between the mirror part (1) and one of N support
cantilevers (2-1 to 2-4). The second axis (the Y axis) is
orthogonal to the first axis (the X axis). This adjustment method
includes the step of applying AC voltage to a plurality of
piezoelectric elements (5i-a to d) by the power supply unit (62).
The phase of AC voltage is a value corresponding to the position of
the piezoelectric element, and the frequency of AC voltage is an
intermediate frequency (Fm) between a resonance frequency F2 in the
first natural frequency mode (natural frequency mode 2) and a
resonance frequency F3 in the second natural frequency mode
(natural frequency mode 3). This adjustment method further includes
the steps of: measuring the deflection angle of the mirror part
(1); and forming a thin film (31, 32) on a part of the mirror part
(1), a part of the support cantilevers (2-1 to 2-4), or a part of
the drive cantilevers (3-1 to 3-4).
Therefore, even when resonant drive with the intermediate frequency
Fm is impossible due to a large difference between the resonance
frequency F2 in the first natural frequency mode (natural frequency
mode 2) and the resonance frequency F3 in the second natural
frequency mode (natural frequency mode 3), a thin film (31, 32) is
formed on a part of the mirror part (1), a part of the support
cantilevers (2-1 to 2-4), or a part of the drive cantilevers (3-1
to 3-4), whereby the difference between the resonance frequency F2
and the resonance frequency F3 is reduced to enable resonant drive
with the intermediate frequency Fm.
(Modifications)
The present invention is not limited to the foregoing embodiments
and includes, for example, modifications as follows.
(A) Shape of Mirror Part 1
In the second to seventh embodiments, mirror part 1 has a
360.degree./N rotational symmetric shape with respect to the Z axis
such that the resonance frequency in the natural frequency mode 2
and the resonance frequency in the natural frequency mode 3 match.
However, embodiments are not limited thereto. Even when mirror part
1 does not have a 360.degree./N rotational symmetric shape with
respect to the Z axis, the resonance frequency in the natural
frequency mode 2 and the resonance frequency in the natural
frequency mode 3 can be matched by adjusting the mechanical
rigidity of the drive cantilevers, the support cantilevers, and the
mirror part. However, the rotation linearity error can be reduced
more when mirror part 1 has a 360.degree./N rotational symmetric
shape with respect to the Z axis.
(B) Number of Piezoelectric Elements
In the first to eleventh embodiments, two piezoelectric elements
are arranged on each of a plurality of circumferential portions
extending in the same direction as the circumferential direction of
mirror part 1 on one drive cantilever. However, embodiments are not
limited thereto. For example, when A, B, C, D, a bend portion, E,
F, G, H piezoelectric elements are arranged on one drive cantilever
in order from an end, AC voltage with a phase P may be applied to
A, B, E, F and AC voltage with a phase (P+180.degree.) may be
applied to C, D, G, H. This reduces mechanical stress exerted on
the piezoelectric elements and alleviates breakage and separation
of the piezoelectric elements. On the other hand, when two
piezoelectric elements are arranged as in the first to eleventh
embodiments, the arrangement area of the piezoelectric elements can
be increased, and the drive force can be increased.
(C) Configuration of Third to Seventh Embodiments
The configuration of the mirror part, the drive cantilevers, the
support cantilevers, and the piezoelectric elements in the third to
seventh embodiments is premised on that the resonance frequency in
the natural frequency mode 2 and the resonance frequency in the
natural frequency mode 3 match. However, embodiments are not
limited thereto. The configuration of the mirror part, the drive
cantilevers, the support cantilevers, and the piezoelectric
elements in the third to seventh embodiments can be applied also
when the intermediate frequency between the resonance frequency in
the natural frequency mode 2 and the resonance frequency in the
natural frequency mode 3 described in the eighth to twelfth
embodiments is used.
Embodiments disclosed here should be understood as being
illustrative rather than being limitative in all respects. The
scope of the present invention is shown not in the foregoing
description but in the claims, and it is intended that all
modifications that come within the meaning and range of equivalence
to the claims are embraced here.
REFERENCE SIGNS LIST
1, 501, 601 mirror part, 1B, 501B, 601B mirror surface, 1C, 501C,
601C silicon mirror part, 2-1 to 2-4, 502-1 to 502-4, 602-1 to
602-3 support cantilever, 3-1 to 3-4, 503-1 to 503-4, 603-1 to
603-3 drive cantilever, 4-1 to 4-4, 504-1 to 504-4, 604-1 to 604-3
fixed portion, 5-1-a to d, 5-2-a to d, 5-3-a to d, 5-4-a to d,
505-1-a to d, 505-2-a to d, 505-3-a to d, 505-4-a to d, 605-1-a to
d, 605-2-a to d, 605-3-a to d piezoelectric element, 6-1 to 6-4
detecting piezoelectric element, 10 SOI substrate, 101 silicon
support layer, 102 silicon active layer, 103 silicon oxide film, 11
insulating film, 12 lower layer electrode, 13 piezoelectric thin
film, 14 upper layer electrode, 15 insulating film, 16 wiring
electrode, 17 package, 18 trimming pattern, 20-1 to 20-4 drive
power supply, 61 control unit, 62 power supply unit, 91 optical
system, 92 target, 93 distance information calculator, 95 receiver
circuit, 96 drive circuit, 99 sensing unit, 100, 200, 300, 400,
500, 600, 700, 800, 900, 1000, 1100, 1200, 1300 optical scanning
device, 31, 32 frequency-adjusting thin film, LD laser diode, PD
photodiode.
* * * * *